Use "tvb_strsize()" rather than "tvb_strnlen()" to get the size

Use "tvb_strsize()" rather than "tvb_strnlen()" to get the size
(including the terminating null byte) of a null-terminated string; it
includes the terminating null in the count, and throws an exception if
the string goes past the end of the tvbuff, so we don't have to do
either of those ourselves.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@3797 f5534014-38df-0310-8fa8-9805f1628bb7