ath5k: check return value of ieee80211_get_tx_rate

This avoids a NULL pointer dereference as reported here:

	https://bugzilla.redhat.com/show_bug.cgi?id=625889

When the WARN condition is hit in ieee80211_get_tx_rate, it will return
NULL.  So, we need to check the return value and avoid dereferencing it
in that case.
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>
Cc: stable@kernel.org
Acked-by: NBob Copeland <me@bobcopeland.com>

drivers/net/wireless/ath/ath5k/base.c

@@ -1327,6 +1327,10 @@ ath5k_txbuf_setup(struct ath5k_softc *sc, struct ath5k_buf *bf,
 			PCI_DMA_TODEVICE);
 
 	rate = ieee80211_get_tx_rate(sc->hw, info);
+	if (!rate) {
+		ret = -EINVAL;
+		goto err_unmap;
+	}
 
 	if (info->flags & IEEE80211_TX_CTL_NO_ACK)
 		flags |= AR5K_TXDESC_NOACK;