Fixing UBSan issues that showed up in all-clusters app (#35580)

* Fix for unsigned integer overflow:
Error Message: BufferWriter.cpp:76:16: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'size_t' (aka 'unsigned long')

when size becomes 0 and we are in while (size-- > 0), then size will become less than 0, which should be avoided since it is an unsigned (although technically it wraps around, UBSAN complains about it).
Fix: stop size from decrement past 0, by moving size-- inside the loop

* Fix null pointer passed to non-null argument in CHIPMemString.h
Error Message: CHIPMemString.h:88:22: runtime error: null pointer passed as argument 2, which is declared to never be null

when PI= is in mDNS TXT record (its value is empty) , and Dnssd::Internal::GetPairingInstruction calls CopyString, source is an empty bytespan and source.data() will return a null pointer, that will be passed to memcpy
Fix: avoid memcpy in that case.

src/lib/support/BufferWriter.cpp

@@ -73,17 +73,19 @@ LittleEndian::BufferWriter & LittleEndian::BufferWriter::EndianPutSigned(int64_t
 
 BigEndian::BufferWriter & BigEndian::BufferWriter::EndianPut(uint64_t x, size_t size)
 {
-    while (size-- > 0)
+    while (size > 0)
     {
+        size--;
         Put(static_cast<uint8_t>((x >> (size * 8)) & 0xff));
     }
     return *this;
 }
 
 BigEndian::BufferWriter & BigEndian::BufferWriter::EndianPutSigned(int64_t x, size_t size)
 {
-    while (size-- > 0)
+    while (size > 0)
     {
+        size--;
         Put(static_cast<uint8_t>((x >> (size * 8)) & 0xff));
     }
     return *this;

src/lib/support/CHIPMemString.h

@@ -82,12 +82,20 @@ inline void CopyString(char (&dest)[N], const char * source)
  */
 inline void CopyString(char * dest, size_t destLength, ByteSpan source)
 {
-    if (dest && destLength)
+    if ((dest == nullptr) || (destLength == 0))
     {
-        size_t maxChars = std::min(destLength - 1, source.size());
-        memcpy(dest, source.data(), maxChars);
-        dest[maxChars] = '\0';
+        return; // no space to copy anything, not even a null terminator
     }
+
+    if (source.empty())
+    {
+        *dest = '\0'; // just a null terminator, we are copying empty data
+        return;
+    }
+
+    size_t maxChars = std::min(destLength - 1, source.size());
+    memcpy(dest, source.data(), maxChars);
+    dest[maxChars] = '\0';
 }
 
 /**