skinny: fix retval, convert from tvb_length

New dissectors return an integer, not a boolean, fix that.

Change-Id: I79dea0cc4274d503d61ce8823dc783c542764f6b
Reviewed-on: https://code.wireshark.org/review/2884
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Evan Huus <eapache@gmail.com>