mac80211: fix possible null-pointer de-reference

This patch not only fixes a null-pointer de-reference
that would be triggered by a PLINK_OPEN frame with mis-
matching/incompatible mesh configuration, but also
responds correctly to non-compatible PLINK_OPEN frames
by generating a PLINK_CLOSE with the right reason code.

The original bug was detected by smatch.
( http://repo.or.cz/w/smatch.git

 )

net/mac80211/mesh_plink.c +574 mesh_rx_plink_frame(168)
error: we previously assumed 'sta' could be null.

Cc: <stable@kernel.org>
Reviewed-and-Tested-by: Steve deRosier <steve@cozybit.com>
Reviewed-and-Tested-by: Javier Cardona <javier@cozybit.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>