mac80211: mesh: handle failed alloc for rmc cache

In the unlikely case that mesh_rmc_init() fails with -ENOMEM,
the rmc pointer will be left as NULL but the interface is still
operational because ieee80211_mesh_init_sdata() is not allowed
to fail.

If this happens, we would blindly dereference rmc when checking
whether a multicast frame is in the cache.  Instead just drop the
frames in the forwarding path.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>