ath9k: fix an uninit value use in ath9k_htc_rx_msg()

ath9k_htc_rx_msg() assumes skb->data contains a full
`struct htc_frame_hdr`, thus it needs a call to pskb_may_pull()
to ensure there is enough data.

This fixes a long-standing issue reported by KMSAN:

  BUG: KMSAN: uninit-value in ath9k_htc_rx_msg+0x544/0x980 drivers/net/wireless/ath/ath9k/htc_hst.c:417
   ath9k_htc_rx_msg+0x544/0x980 drivers/net/wireless/ath/ath9k/htc_hst.c:417
   ath9k_hif_usb_rx_stream drivers/net/wireless/ath/ath9k/hif_usb.c:653 [inline]
   ath9k_hif_usb_rx_cb+0x196a/0x1f10 drivers/net/wireless/ath/ath9k/hif_usb.c:686
   __usb_hcd_giveback_urb+0x522/0x740 drivers/usb/core/hcd.c:1670
   usb_hcd_giveback_urb+0x150/0x620 drivers/usb/core/hcd.c:1747
   dummy_timer+0xd3f/0x4f20 drivers/usb/gadget/udc/dummy_hcd.c:1988
   call_timer_fn+0x43/0x480 kernel/time/timer.c:1474
   expire_timers+0x272/0x610 kernel/time/timer.c:1519
   __run_timers+0x5bc/0x8c0 kernel/time/timer.c:1790
  ...

  Uninit was created at:
  ...
   __alloc_skb+0x34a/0xd70 net/core/skbuff.c:426
   __netdev_alloc_skb+0x126/0x780 net/core/skbuff.c:494
   __dev_alloc_skb include/linux/skbuff.h:3264 [inline]
   ath9k_hif_usb_rx_stream drivers/net/wireless/ath/ath9k/hif_usb.c:635 [inline]
   ath9k_hif_usb_rx_cb+0xe7b/0x1f10 drivers/net/wireless/ath/ath9k/hif_usb.c:686
   __usb_hcd_giveback_urb+0x522/0x740 drivers/usb/core/hcd.c:1670
  ...

Reported-by: syzbot+2ca247c2d60c7023de7f@syzkaller.appspotmail.com
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Signed-off-by: Alexander Potapenko <glider@google.com>

drivers/net/wireless/ath/ath9k/htc_hst.c

@@ -408,7 +408,8 @@ void ath9k_htc_rx_msg(struct htc_target *htc_handle,
 	struct htc_endpoint *endpoint;
 	__be16 *msg_id;
 
-	if (!htc_handle || !skb)
+	if (!htc_handle || !skb ||
+	    !pskb_may_pull(skb, sizeof(struct htc_frame_hdr)))
 		return;
 
 	htc_hdr = (struct htc_frame_hdr *) skb->data;