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

Manually merged from ede5a9d4c0364c184bb79786128244343b0abe06  (master branch)

bug:10281
Change-Id: I8d4ba699b8244793cdffe5d72a78ccfefe789e94
Reviewed-on: https://code.wireshark.org/review/3269
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Michael Mann <mmann78@netscape.net>