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>