rtlwifi: Handle previous allocation failures when freeing device memory

Handle previous allocation failures when freeing device memory

Signed-off-by: Simon Graham <simon.graham@virtualcomputer.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>