As suggested by Chris: use val_to_str_const() instead of match_strval() plus a

As suggested by Chris: use val_to_str_const() instead of match_strval() plus a
NULL-return check.

Use val_to_str_const instead of val_to_str() in a couple places where the string
is constant.

Use val_to_str() instead of blindly passing the return value from match_strval()
into a format routine (to ensure a non-NULL string pointer).  A couple of these
were cases where it could not actually return NULL, but I changed it for
consistency.

Store the return value of match_strval() rather than calling it repeatedly.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@37204 f5534014-38df-0310-8fa8-9805f1628bb7