Our implementation of vsnprintf for systems where it is missing did not
correctly return a negative value on error, instead returning the size
passed in. We got this code from tcpdump/libpcap, and it was wrong
there, too, though their latest master branch has removed it in favor of
requiring a C99 compiler (C99 guarantees vsnprintf).
This should remove a LGTM code analysis finding (See #1834) of
cpp/constant-comparison in Ncat because we were checking for a negative
return from Snprintf, which would never occur.
3c7f2de01b