Add cast to satisfy win7 buildbot

I really don't understand why MSVC would make the result of this computation an
int64 then complain about down-casting to an int16 when *all* of the
participating variables are int16 or smaller...

Change-Id: I2d9c27ac22b51b10e4872a6640881c8d0ec566e7
Reviewed-on: https://code.wireshark.org/review/3180
Reviewed-by: Evan Huus <eapache@gmail.com>