tcp: fix null ptr deref in getsockopt(..., TCP_ULP, ...)

If icsk_ulp_ops is unset, it dereferences a null ptr.
Add a null ptr check.

BUG: KASAN: null-ptr-deref in copy_to_user include/linux/uaccess.h:168 [inline]
BUG: KASAN: null-ptr-deref in do_tcp_getsockopt.isra.33+0x24f/0x1e30 net/ipv4/tcp.c:3057
Read of size 4 at addr 0000000000000020 by task syz-executor1/15452

Signed-off-by: Dave Watson <davejwatson@fb.com>
Reported-by: "Levin, Alexander (Sasha Levin)" <alexander.levin@verizon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -3062,6 +3062,11 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
 		if (get_user(len, optlen))
 			return -EFAULT;
 		len = min_t(unsigned int, len, TCP_ULP_NAME_MAX);
+		if (!icsk->icsk_ulp_ops) {
+			if (put_user(0, optlen))
+				return -EFAULT;
+			return 0;
+		}
 		if (put_user(len, optlen))
 			return -EFAULT;
 		if (copy_to_user(optval, icsk->icsk_ulp_ops->name, len))