NFC: trf7970a: Add NULL check to clear up smatch warning

Although it should be unnecessary, add a NULL pointer check
to trf7970a_send_upstream() to eliminate a smatch warning.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
diff --git a/drivers/nfc/trf7970a.c b/drivers/nfc/trf7970a.c
index 08f23d7..70b0707 100644
--- a/drivers/nfc/trf7970a.c
+++ b/drivers/nfc/trf7970a.c
@@ -629,7 +629,9 @@
 	}
 
 	if (trf->adjust_resp_len) {
-		skb_trim(trf->rx_skb, trf->rx_skb->len - 1);
+		if (trf->rx_skb)
+			skb_trim(trf->rx_skb, trf->rx_skb->len - 1);
+
 		trf->adjust_resp_len = false;
 	}