Fix a nasty side-effect of running the packet list in

Fix a nasty side-effect of running the packet list in
GTK_SELECTION_BROWSE mode - that mode apparently always arranges that
there is (in a non-empty GtkCList) one row selected, which means that
when the first row is added, it selects it.

Unfortunately, that causes a "select-row" signal to be emitted, which
causes "select_packet()" to be called - but we haven't yet set "fd->row"
for the frame we're adding, so "select_packet()" can't find the frame
for the row being selected, and it aborts.

This causes a core dump when the first packet arrives during an "Update
list of packets in real time" capture.

For now, we handle this by, if we don't find the frame with the given
row number, checking that there's exactly one frame in our list of
frames and, if so, saying that frame is the frame for which we're
looking.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@1409 f5534014-38df-0310-8fa8-9805f1628bb7