nl80211: fix null-ptr dereference on invalid mesh configuration

If TX rates are specified during mesh join, the channel must
also be specified. Check the channel pointer to avoid a null
pointer dereference if it isn't.

Reported-by: Jouni Malinen <j@w1.fi>
Fixes: 8564e382 ("cfg80211: add checks for beacon rate, extend to mesh")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Git-commit: 265698d7e6132a2d41471135534f4f36ad15b09c
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


CRs-fixed: 2138848
Change-Id: I6a9d89181c13034fff3fbbbca953f0c9fdc520c9
Signed-off-by: Peng Xu <pxu@codeaurora.org>