net: usb: sr9700: Handle negative len

Packet len computed as difference of length word extracted from
skb data and four may result in a negative value. In such case
processing of the buffer should be interrupted rather than
setting sr_skb->len to an unexpectedly large value (due to cast
from signed to unsigned integer) and passing sr_skb to
usbnet_skb_return.

Fixes: e9da0b56fe27 ("sr9700: sanity check for packet length")
Signed-off-by: Szymon Heidrich <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

drivers/net/usb/sr9700.c

···
 		/* ignore the CRC length */
 		len = (skb->data[1] | (skb->data[2] << 8)) - 4;
 
-		if (len > ETH_FRAME_LEN || len > skb->len)
+		if (len > ETH_FRAME_LEN || len > skb->len || len < 0)
 			return 0;
 
 		/* the last packet of current skb */