Squelch some compiler warnings.

The version of GCC on the OS X 32-bit buildbot isn't smart enough to
figure out that this can't happen (it's one of those "if (xxx) foo =
bar; ... if (xxx) use foo;" cases.)

Change-Id: I04fef2d602c913761ae7832c4f568aaaad398c87
Reviewed-on: https://code.wireshark.org/review/3390
Reviewed-by: Guy Harris <guy@alum.mit.edu>