mei: protect mei_cl_mtu from null dereference

A receive callback is queued while the client is still connected
but can still be called after the client was disconnected. Upon
disconnect cl->me_cl is set to NULL, hence we need to check
that ME client is not-NULL in mei_cl_mtu to avoid
null dereference.

Cc: <stable@vger.kernel.org>
Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Link: https://lore.kernel.org/r/20201029095444.957924-2-tomas.winkler@intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/mei/client.h

@@ -182,11 +182,11 @@ static inline u8 mei_cl_me_id(const struct mei_cl *cl)
  *
  * @cl: host client
  *
- * Return: mtu
+ * Return: mtu or 0 if client is not connected
  */
 static inline size_t mei_cl_mtu(const struct mei_cl *cl)
 {
-	return cl->me_cl->props.max_msg_length;
+	return cl->me_cl ? cl->me_cl->props.max_msg_length : 0;
 }
 
 /**