remove unnecessary integer overflow checks

... and don't call THROW() from a dissector

If we have a guint32 a and interpret 2*a as gint, we'll always end up
with a negative value if 2*a overflows. Both tvb_get_string_enc() and
proto_tree_add_item() handle this case and throw an exception.

Change-Id: Ibb142328837b6a583057531c76d08631fc731848
Reviewed-on: https://code.wireshark.org/review/5948
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>