net/x25: Fix skb leak in x25_lapb_receive_frame()
In the Linux kernel, the following vulnerability has been resolved:

net/x25: Fix skb leak in x25_lapb_receive_frame()

x25_lapb_receive_frame() using skb_copy() to get a private copy of
skb, the new skb should be freed in the undersized/fragmented skb
error handling path. Otherwise there is a memory leak.