rtlwifi: Fix oops on rate-control failure

When the rate-control indexing is incorrectly set up, mac80211 issues
a warning and returns NULL from the call to ieee80211_get_tx_rate().
When this happens, avoid a NULL pointer dereference.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>