ms-mms: fix retval for short pkts, fix reading flags

uint16 & 0xFF00 will always equal 0. Use tvb_get_guint8 instead. Convert
from tvb_length to tvb_captured_length.

Fix the return value of dissectors, 0 means reject but that cannot be
combined with reassembly requests.

Change-Id: I5fca66e2e809699392237aff5813eecdfb15857f
Reviewed-on: https://code.wireshark.org/review/2885
Reviewed-by: Evan Huus <eapache@gmail.com>