Store the result of tvb_get_ntohs() in a guint16 instead of a guint8 to avoid a

Store the result of tvb_get_ntohs() in a guint16 instead of a guint8 to avoid a
compiler warning.  (It looks like the only expected values will fit in 8 bits,
but it also appears that the code really is expecting 'of_code' to be stored in
16 bits on the wire.)

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@35790 f5534014-38df-0310-8fa8-9805f1628bb7