Use "tvb_offset_exists()" rather than "tvb_length_remaining()" in a loop

Use "tvb_offset_exists()" rather than "tvb_length_remaining()" in a loop
scanning the tvbuff.  Also use "tvb_offset_exists()" before accessing
the tvbuff in that loop; the loop is only building the Info column, so
it shouldn't throw an exception before building the list.

(XXX - it should arguably all be done in one loop, with the Info column
built on the fly.)

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@4672 f5534014-38df-0310-8fa8-9805f1628bb7