The following error occurs on Windows when attempting to use string_replace function though may happen on some other exotic platform:

file:..\..\..\src\libs\zbxcommon\str.c,line:388 zbx_malloc: out of memory. Requested 4294967614 bytes.

Diff is signed, while count is unsigned.

long diff;
unsigned long count = 0; 

Diff can be negative:

diff = (long)strlen(sub_str2) - len;        /* allocate new memory */

Diff is add to strlen, however count is unsigned so count*diff may become large number in my case 4294967614 which is larger then maximum unsigned long type.

	new_str = (char *)zbx_malloc(new_str, (size_t)(strlen(str) + count*diff + 1)*sizeof(char));