Fix some compilation errors (size_t to guint/gint) and an unused parameter

Change-Id: I2d64d840c93f951dd5481ca5950ff5587da6843d
Reviewed-on: https://code.wireshark.org/review/5112
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Guy Harris <guy@alum.mit.edu>