Bluetooth: 6lowpan: Avoid memory leak if memory allocation fails

If skb_unshare() returns NULL, then we leak the original skb.
Solution is to use temp variable to hold the new skb.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>