fix underflow in BER constrained bitstrings

this can happen and cause invalid memory accesses with incorrectly-large padding
values

Conflicts:
	epan/dissectors/packet-ber.c

Bug:10187
Change-Id: Ib9b2a2fa10766efb4d95d588f57354a56373c626
Reviewed-on: https://code.wireshark.org/review/2328
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-on: https://code.wireshark.org/review/3438
Reviewed-by: Balint Reczey <balint@balintreczey.hu>