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.

doc/fixes34.4

@@ -271,6 +271,7 @@ prevent very large number of objects in # inventory slot from causing
 	buffer overflow
 !fixinv config was using arbitrary characters instead of # for invent overflow
 don't report death by petrification if cockatrice kills hero via HP loss
+Riders are immune to green slime
 
 
 Platform- and/or Interface-Specific Fixes

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,