fix for breaking a crystal helm

Avoid impossible warning "breaking odd object?" when throwing a helm
of brilliance causes it to break.  Presumably dropping one while
levitating or zapping one of the floor with striking/force bolt would
yield similar result.

While in there, reduce the chance of breakage from 99% to 5% for both
helm of brilliance and crystal plate mail.

doc/fixes3-7-0.txt

@@ -1554,6 +1554,7 @@ if hero was killed by a wand zapped by a monster, cause of death was reported
 throttle the difficulty of the monsters in theme room "buried zombies"
 buried troll whose auto-revive timer expired might triger panic with
         "revive default case 6"
+throwing a helm of brilliance could yield "breaking odd object?"
 
 
 Fixes to 3.7.0-x Platform and/or Interface Problems Exposed Via git Repository

src/dothrow.c

@@ -2458,7 +2458,14 @@ breakobj(
 boolean
 breaktest(struct obj *obj)
 {
-    if (obj_resists(obj, 1, 99))
+    int nonbreakchance = 1; /* chance for non-artifacts to resist */
+
+    /* this may need to be changed if actual glass armor gets added someday;
+       for now, it affects crystal plate mail and helm of brilliance */
+    if (obj->oclass == ARMOR_CLASS)
+        nonbreakchance = 95;
+
+    if (obj_resists(obj, nonbreakchance, 99))
         return 0;
     if (objects[obj->otyp].oc_material == GLASS && !obj->oartifact
         && obj->oclass != GEM_CLASS)
@@ -2486,9 +2493,10 @@ breakmsg(struct obj *obj, boolean in_view)
     switch (obj->oclass == POTION_CLASS ? POT_WATER : obj->otyp) {
     default: /* glass or crystal wand */
         if (obj->oclass != WAND_CLASS)
-            impossible("breaking odd object?");
+            impossible("breaking odd object (%d)?", obj->otyp);
         /*FALLTHRU*/
     case CRYSTAL_PLATE_MAIL:
+    case HELM_OF_BRILLIANCE:
     case LENSES:
     case MIRROR:
     case CRYSTAL_BALL: