Fix BigInt#to_s emitting null bytes for certain values

spec/std/big/big_decimal_spec.cr

@@ -318,6 +318,7 @@ describe BigDecimal do
     BigDecimal.new(0).to_s.should eq "0"
     BigDecimal.new(1).to_s.should eq "1"
     BigDecimal.new(-1).to_s.should eq "-1"
+    BigDecimal.new("8.5").to_s.should eq "8.5"
     BigDecimal.new("-0.35").to_s.should eq "-0.35"
     BigDecimal.new("-.35").to_s.should eq "-0.35"
     BigDecimal.new("0.01").to_s.should eq "0.01"

spec/std/big/big_int_spec.cr

@@ -392,11 +392,27 @@ describe "BigInt" do
       it_converts_to_s 2.to_big_i, "00002", precision: 5
       it_converts_to_s 2.to_big_i, "#{"0" * 199}2", precision: 200
 
-      it_converts_to_s -1.to_big_i, "-1", precision: 0
-      it_converts_to_s -1.to_big_i, "-1", precision: 1
-      it_converts_to_s -1.to_big_i, "-01", precision: 2
-      it_converts_to_s -1.to_big_i, "-00001", precision: 5
-      it_converts_to_s -1.to_big_i, "-#{"0" * 199}1", precision: 200
+      it_converts_to_s (-1).to_big_i, "-1", precision: 0
+      it_converts_to_s (-1).to_big_i, "-1", precision: 1
+      it_converts_to_s (-1).to_big_i, "-01", precision: 2
+      it_converts_to_s (-1).to_big_i, "-00001", precision: 5
+      it_converts_to_s (-1).to_big_i, "-#{"0" * 199}1", precision: 200
+
+      it_converts_to_s 85.to_big_i, "85", precision: 0
+      it_converts_to_s 85.to_big_i, "85", precision: 1
+      it_converts_to_s 85.to_big_i, "85", precision: 2
+      it_converts_to_s 85.to_big_i, "085", precision: 3
+      it_converts_to_s 85.to_big_i, "0085", precision: 4
+      it_converts_to_s 85.to_big_i, "00085", precision: 5
+      it_converts_to_s 85.to_big_i, "#{"0" * 198}85", precision: 200
+
+      it_converts_to_s (-85).to_big_i, "-85", precision: 0
+      it_converts_to_s (-85).to_big_i, "-85", precision: 1
+      it_converts_to_s (-85).to_big_i, "-85", precision: 2
+      it_converts_to_s (-85).to_big_i, "-085", precision: 3
+      it_converts_to_s (-85).to_big_i, "-0085", precision: 4
+      it_converts_to_s (-85).to_big_i, "-00085", precision: 5
+      it_converts_to_s (-85).to_big_i, "-#{"0" * 198}85", precision: 200
 
       it_converts_to_s 123.to_big_i, "123", precision: 0
       it_converts_to_s 123.to_big_i, "123", precision: 1

src/big/big_int.cr

@@ -429,7 +429,25 @@ struct BigInt < Int
       if precision <= count
         len = count + (negative ? 1 : 0)
         String.new(len + 1) do |buffer| # null terminator required by GMP
+          buffer[len - 1] = 0
           LibGMP.get_str(buffer, upcase ? -base : base, self)
+
+          # `sizeinbase` may be 1 greater than the exact value
+          if buffer[len - 1] == 0
+            if precision == count
+              # In this case the exact `count` is `precision - 1`, i.e. one zero
+              # should be inserted at the beginning of the number
+              # e.g. precision = 3, count = 3, exact count = 2
+              # "85\0\0" -> "085\0" for positive
+              # "-85\0\0" -> "-085\0" for negative
+              start = buffer + (negative ? 1 : 0)
+              start.move_to(start + 1, count - 1)
+              start.value = '0'.ord.to_u8
+            else
+              len -= 1
+            end
+          end
+
           base62_swapcase(Slice.new(buffer, len)) if base == 62
           {len, len}
         end
@@ -439,14 +457,28 @@ struct BigInt < Int
           # e.g. precision = 13, count = 8
           # "_____12345678\0" for positive
           # "_____-12345678\0" for negative
-          LibGMP.get_str(buffer + precision - count, upcase ? -base : base, self)
+          buffer[len - 1] = 0
+          start = buffer + precision - count
+          LibGMP.get_str(start, upcase ? -base : base, self)
+
+          # `sizeinbase` may be 1 greater than the exact value
+          if buffer[len - 1] == 0
+            # e.g. precision = 7, count = 3, exact count = 2
+            # "____85\0\0" -> "____885\0" for positive
+            # "____-85\0\0" -> "____-885\0" for negative
+            # `start` will be zero-filled later
+            count -= 1
+            start += 1 if negative
+            start.move_to(start + 1, count)
+          end
+
           base62_swapcase(Slice.new(buffer + len - count, count)) if base == 62
 
           if negative
             buffer.value = '-'.ord.to_u8
             buffer += 1
           end
-          Intrinsics.memset(buffer, '0'.ord.to_u8, precision - count, false)
+          Slice.new(buffer, precision - count).fill('0'.ord.to_u8)
 
           {len, len}
         end
@@ -471,6 +503,9 @@ struct BigInt < Int
       ptr = LibGMP.get_str(nil, upcase ? -base : base, self)
       negative = self < 0
 
+      # `sizeinbase` may be 1 greater than the exact value
+      count -= 1 if ptr[count + (negative ? 0 : -1)] == 0
+
       if precision <= count
         buffer = Slice.new(ptr, count + (negative ? 1 : 0))
       else