res_timing: Don't close FD 0 when out of open files.

This fixes so a failure to get a timer file descriptor does not cascade
to closing FD 0.

On error, both res_timing_kqueue and res_timing_timerfd would call the
destructor before setting the file handle. The file handle had been
initialized to 0, causing FD 0 to be closed. This in turn, resulted in
floods of "CLI>" messages and an unusable terminal.

ASTERISK-19277 #close
Reported by: Barry Chern

For the master branch, this was already fixed. This patch only ensures
that we do not attempt to close a negative file descriptor.

Change-Id: I147d7e33726c6e5a2751928d56561494f5800350