ipfw(8) crashes when ext6hdr option is omitted

Verify that the option is passed, error out if it's not.
The problem can be trivially triggered with `ipfw add allow ext6hdr`.

PR:		253169
Reviewed by:	kp@
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D28447

sbin/ipfw/ipfw2.c

@@ -4961,6 +4961,7 @@ compile_rule(char *av[], uint32_t *rbuf, int *rbufsize, struct tidx *tstate)
 			break;
 
 		case TOK_EXT6HDR:
+			NEED1("missing extension header");
 			fill_ext6hdr( cmd, *av );
 			av++;
 			break;