tcp: no md5sig option size check bug

tcp_parse_md5sig_option doesn't check md5sig option (TCPOPT_MD5SIG)
length, but tcp_v[46]_inbound_md5_hash assume that it's at least 16
bytes long.

Signed-off-by: Dmitry Popov <dp@highloadlab.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -3930,7 +3930,7 @@ u8 *tcp_parse_md5sig_option(struct tcphdr *th)
 			if (opsize < 2 || opsize > length)
 				return NULL;
 			if (opcode == TCPOPT_MD5SIG)
-				return ptr;
+				return opsize == TCPOLEN_MD5SIG ? ptr : NULL;
 		}
 		ptr += opsize - 2;
 		length -= opsize;