Convert tvb_get_ptr buffer access macros to use much safer tvb access macros.

(Manually merged from master-1.12 branch)

bug:10281
Change-Id: Ie0818b3c21a802fe6d597197dae0075162d80ded
Reviewed-on: https://code.wireshark.org/review/3270
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Michael Mann <mmann78@netscape.net>