Micro-optimize tvbuff:

 - compute_offset_and_remaining(...) is a little bit faster than
   check_offset_length(.., .length_val =  -1);

 - tvb_find_guint8(), tvb_pbrk_guint8() - both function are limiting data to
   MIN((unsigned) maxlength, tvb_len) - do the same with less count of ifs.

Change-Id: I7761d77b6282d800eea94852d5c6543aef4bc7ca
Reviewed-on: https://code.wireshark.org/review/2829
Reviewed-by: Anders Broman <a.broman58@gmail.com>