va_list can't be NULL on ARM.

Bug: 10209
Change-Id: Ibd63a530450b7d2d4ec244e91c77caa731ba63aa
Signed-off-by: Steev Klimaszewski <threeway@gmail.com>
Signed-off-by: Balint Reczey <balint@balintreczey.hu>
Reviewed-on: https://code.wireshark.org/review/2464
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>