Fix the handling of invalid parameters in ASCONF chunks.

Thanks to Mark Wodrich from Google for reporting the issue in
#376
for the userland stack.

usrsctplib/netinet/sctp_asconf.c

@@ -34,7 +34,7 @@
 
 #ifdef __FreeBSD__
 #include <sys/cdefs.h>
-__FBSDID("$FreeBSD: head/sys/netinet/sctp_asconf.c 350745 2019-08-08 08:23:27Z tuexen $");
+__FBSDID("$FreeBSD: head/sys/netinet/sctp_asconf.c 352550 2019-09-20 08:20:20Z tuexen $");
 #endif
 
 #include <netinet/sctp_os.h>
@@ -720,6 +720,7 @@ sctp_handle_asconf(struct mbuf *m, unsigned int offset,
 		if (param_length <= sizeof(struct sctp_paramhdr)) {
 			SCTPDBG(SCTP_DEBUG_ASCONF1, "handle_asconf: param length (%u) too short\n", param_length);
 			sctp_m_freem(m_ack);
+			return;
 		}
 		/* get the entire parameter */
 		aph = (struct sctp_asconf_paramhdr *)sctp_m_getptr(m, offset, param_length, aparam_buf);