Validate no_of_bits in ws_sign_ext32 and ws_sign_ext64

The result of the '<<' expression is undefined if no_of_bits - 1 is negative.

Change-Id: I3fff09afe414bdd9d6736ee351f1c542c503b93d
Reviewed-on: https://code.wireshark.org/review/4698
Reviewed-by: Evan Huus <eapache@gmail.com>
Petri-Dish: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>