Squelch a compiler warning.

Change-Id: I8916bb3cc136c03c39fbdaf5d7bb61c3dae137e6
Reviewed-on: https://code.wireshark.org/review/3422
Reviewed-by: Guy Harris <guy@alum.mit.edu>
(cherry picked from commit ce7b041305330e2cb87d895f57381241702c9d85)
Reviewed-on: https://code.wireshark.org/review/3423