Correctly handle negative floats

Fixes: #114

big.go

@@ -514,11 +514,12 @@ func (x *Big) Float64() (f float64, ok bool) {
 	case xc == 0:
 		ok = true
 	case x.IsInt():
-		if xc, ok = x.Uint64(); !ok {
-			v, _ := x.Int64()
-			xc = uint64(v)
+		if xc, ok := x.Int64(); ok {
+			f = float64(xc)
+		} else if xc, ok := x.Uint64(); ok {
+			f = float64(xc)
 		}
-		fallthrough
+		ok = xc < maxMantissa || (xc&(xc-1)) == 0
 	case x.exp == 0:
 		f = float64(xc)
 		ok = xc < maxMantissa || (xc&(xc-1)) == 0

issues_test.go

@@ -83,6 +83,17 @@ func TestIssue105(t *testing.T) {
 	var b Big
 	b.SetString("6190.000000000000")
 	if _, ok := b.Float64(); !ok {
-		t.Fatalf("6190 should fit in a float just fine")
+		t.Fatal("6190 should fit in a float just fine")
+	}
+}
+
+func TestIssue114(t *testing.T) {
+	val := New(-1, 0)
+	f, ok := val.Float64()
+	if !ok {
+		t.Fatal("expected true, got false")
+	}
+	if f != -1 {
+		t.Fatalf("expected -1, got %f", f)
 	}
 }