Fix: Check always for skb in mptcp_is_available

mptcp_is_available may get called without an skb - Thus we have to be
sure that it exists.

Fix provided by Per Hurtig.

Reported-by: Per Hurtig <per.hurtig@kau.se>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>