In "fragment_add_seq_check()", handle short frames the same way

In "fragment_add_seq_check()", handle short frames the same way
regardless of whether they're the first frame we see in a reassembly or
not - put all but the last fragment into the hash table (so subsequent
frames with the same reassembly ID don't get misdissected as
unfragmented frames), return the head of the fragment list for the first
fragment and NULL for all other fragments (so the first fragment gets
dissected as a fragmented packet), and unhash the fragment head from the
reassembly hash table when we see the last fragment.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@5189 f5534014-38df-0310-8fa8-9805f1628bb7