mISDN: Fix a use after free in hfcmulti_tx()

Don't dereference *sp after calling dev_kfree_skb(*sp).

Fixes: af69fb3 ("Add mISDN HFC multiport driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/8be65f5a-c2dd-4ba0-8a10-bfe5980b8cfb@stanley.mountain
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Dan Carpenter

drivers/isdn/hardware/mISDN/hfcmulti.c

@@ -1901,7 +1901,7 @@ hfcmulti_dtmf(struct hfc_multi *hc)
 static void
 hfcmulti_tx(struct hfc_multi *hc, int ch)
 {
-	int i, ii, temp, len = 0;
+	int i, ii, temp, tmp_len, len = 0;
 	int Zspace, z1, z2; /* must be int for calculation */
 	int Fspace, f1, f2;
 	u_char *d;
@@ -2122,14 +2122,15 @@ hfcmulti_tx(struct hfc_multi *hc, int ch)
 		HFC_wait_nodebug(hc);
 	}
 
+	tmp_len = (*sp)->len;
 	dev_kfree_skb(*sp);
 	/* check for next frame */
 	if (bch && get_next_bframe(bch)) {
-		len = (*sp)->len;
+		len = tmp_len;
 		goto next_frame;
 	}
 	if (dch && get_next_dframe(dch)) {
-		len = (*sp)->len;
+		len = tmp_len;
 		goto next_frame;
 	}