Clean up another signed vs. unsigned comparison warning - if

Clean up another signed vs. unsigned comparison warning - if
"snprintf()" returns a negative number, that's an error, and we assume
"errno" was set and return NULL, otherwise we cast its return value to
"size_t" and compare it with the size of the buffer we were given, and,
if it was bigger, we know that "snprintf()" didn't generate all the
characters it could be cause they wouldn't have fit, so we set "errno"
to ENOSPC and return NULL.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@4095 f5534014-38df-0310-8fa8-9805f1628bb7