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: I2a973b6168235d5d1c7f2a5f8ac79b97b963d846
Reviewed-on: https://code.wireshark.org/review/2863
Reviewed-by: Guy Harris <guy@alum.mit.edu>