Fix an error caught by enum-conversion warnings.

(Strong typing is for weak minds.

Human minds are weak.

Therefore, strong typing is for human minds.)

Change-Id: I099b85e98f3b9742b1addd8d260b3e94ca7add31
Reviewed-on: https://code.wireshark.org/review/2866
Reviewed-by: Guy Harris <guy@alum.mit.edu>