Don't peek inside the column information and manually wrap "GTP < ... >"

Don't peek inside the column information and manually wrap "GTP < ... >"
around the protocol field, use the standard routines for prepending to
and appending to a column.  That should fix bug 2311, as the standard
routines use g_strlcat(), and should also keep that code robust in the
face of changes to the way columns are implemented.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@24502 f5534014-38df-0310-8fa8-9805f1628bb7