Rider strengthening

High resistance made the Riders unlikely to be polymorphed, but
they were susceptible to being turned into green slime (and then never
reviving if killed in that state).  Now they'll be immune to both types
of transformation.

src/mon.c

@@ -2584,6 +2584,9 @@ boolean msg;		/* "The oldmon turns into a newmon!" */
 	struct permonst *olddata = mtmp->data;
 	char oldname[BUFSZ], newname[BUFSZ];
 
+	/* Riders are immune to polymorph and green slime */
+	if (is_rider(mtmp->data)) return 0;
+
 	if (msg) {
 	    /* like Monnam() but never mention saddle */
 	    Strcpy(oldname, x_monnam(mtmp, ARTICLE_THE, (char *)0,