tcpdump: cope with incorrect packet lengths

It's possible for the capture buffer to be smaller than indicated by the
header length. However, pfsync_print() only took the header length into
account. As a result we could read outside of the buffer.

Check that we have at least the expected amount of data before we start
parsing.

PR:		278034
MFC after:	2 weeks
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D44580

contrib/tcpdump/print-pfsync.c

@@ -86,7 +86,7 @@ pfsync_ip_print(netdissect_options *ndo , const u_char *bp, u_int len)
 {
 	struct pfsync_header *hdr = (struct pfsync_header *)bp;
 
-	if (len < PFSYNC_HDRLEN)
+	if (len < PFSYNC_HDRLEN || !ND_TTEST_LEN(bp, len))
 		ND_PRINT("[|pfsync]");
 	else
 		pfsync_print(ndo, hdr, bp + sizeof(struct pfsync_header),