Fix the fuzz failure reported (by me) in https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=8764 :
Apply the fix from bug 8539 (r48796) to another function (dissect_r3_upstreamcommand_queryconfig()):
Bail out of the item length we get (which we use to increment the offset) is 0.
Otherwise the offset does not advance and we loop forever.
While we're in there: get the item length just once (there's no need to call
tvb_get_guint8() a half dozen times when one will do).