dothrow: Check for LOW_PM before accessing mon array

on toss_up() when checking for touch_petrifies, add
a check for corpsenm >= LOW_PM as EGG can be without
corpse reference.

This prevents:
dothrow.c:1266:29: runtime error: index -1 out of bounds for type 'permonst [384]'

src/dothrow.c

@@ -1263,6 +1263,7 @@ toss_up(struct obj *obj, boolean hitsroof)
     const char *action;
     int otyp = obj->otyp;
     boolean petrifier = ((otyp == EGG || otyp == CORPSE)
+                         && obj->corpsenm >= LOW_PM
                          && touch_petrifies(&mons[obj->corpsenm]));
     /* note: obj->quan == 1 */