Message tweak for faulty amulet of life saving

Remove the second reference to "your medallion" from "Your medallion
begins to glow! But... the chain on your medallion breaks..." as being
redundant.

While here, I also strengthened the checks in faulty_lifesaver() - null
object case didn't return, so it would have caused a segfault after
printing an impossible, and I felt like it needed an explicit check that
the object is actually an amulet of life saving, since that might (but
isn't likely to) change in the future.

src/end.c

@@ -1376,7 +1376,7 @@ done(int how)
         Your("medallion %s!", !Blind ? "begins to glow" : "feels warm");
         /* It's cursed? Well, that's just too bad. */
         if (faulty_lifesaver(uamul)) {
-            pline("But... the chain on your medallion breaks and it falls to the %s!",
+            pline("But... the chain breaks and it falls to the %s!",
                   surface(u.ux, u.uy));
             You_hear(Hallucination ? "someone playing Yakety Sax!"
                                    : "the sound of a distant trombone...");

src/mon.c

@@ -2899,6 +2899,15 @@ faulty_lifesaver(struct obj* obj)
 {
     if (!obj) {
         impossible("faulty_lifesaver with null object");
+        return FALSE;
+    }
+    if (obj->otyp != AMULET_OF_LIFE_SAVING) {
+        /* amulets of LS being the only thing that provides Lifesaved could
+         * theoretically change at some point; if so this condition will need to
+         * be adjusted */
+        impossible("faulty_lifesaver with non-amulet-of-life-saving %d",
+                   obj->otyp);
+        return FALSE;
     }
     if (objects[obj->otyp].oc_oprop != LIFESAVED) {
         impossible("faulty_lifesaver with non-life-saving object");