Squelch another warning.

(Yes, we should, on platforms with a 32-bit time_t, check to make sure
the time stamp fits and do something if it doesn't.  Or we should make
the seconds part of an nstime_t be 64-bit and handle overly-large values
when converting them to year/month/day/hour/minute/second.)

Change-Id: If219534985dce29d00754ff151f6c4b5893080d8
Reviewed-on: https://code.wireshark.org/review/4675
Reviewed-by: Guy Harris <guy@alum.mit.edu>