tipc: fix dereference before check warning

This fixes the following Smatch warning:
net/tipc/link.c:2364 tipc_link_recv_fragment()
    warn: variable dereferenced before check '*head' (see line 2361)

A null pointer might be passed to skb_try_coalesce if
a malicious sender injects orphan fragments on a link.

Signed-off-by: Erik Hugne <erik.hugne@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>