Fixed: FString::ReallocBuffer could write to unallocated memory

- Previously, calling ReallocBuffer with a smaller buffer size than the
  current one could overwrite unallocated memory. This required that the
  string it was called on had more than one reference and therefore
  required creating a new copy. The entire original string would be
  copied, whether it fit in the new buffer or not.

src/zstring.cpp

@@ -1096,7 +1096,7 @@ void FString::ReallocBuffer (size_t newlen)
 	{ // If more than one reference, we must use a new copy
 		FStringData *old = Data();
 		AllocBuffer (newlen);
-		StrCopy (Chars, old->Chars(), old->Len);
+		StrCopy (Chars, old->Chars(), newlen < old->Len ? newlen : old->Len);
 		old->Release();
 	}
 	else