brcmfmac: Fix crash on cleanup.

When driver gets unloaded due to error situation there is a chance
a packet gets received while fws has already been cleaned up. This
will result in kernel crash. This patch adds a check to avoid this
crash.
Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/brcm80211/brcmfmac/fwsignal.c

@@ -1636,7 +1636,7 @@ int brcmf_fws_hdrpull(struct brcmf_pub *drvr, int ifidx, s16 signal_len,
 	if (!signal_len)
 		return 0;
 	/* if flow control disabled, skip to packet data and leave */
-	if (!fws->fw_signals) {
+	if ((!fws) || (!fws->fw_signals)) {
 		skb_pull(skb, signal_len);
 		return 0;
 	}