Bugpaper: check if uamul is null before trying to dereference it.

Some that reads through inventory directly (like theft) sometimes hits objects that claim to be worn amulets but aren't.

This is probably a bad sign, but we can deal with it more gracefully than crashes.

src/do_wear.c

@@ -911,7 +911,11 @@ void
 Amulet_off()
 {
     takeoff_mask &= ~W_AMUL;
-
+	
+	if(!uamul){
+		impossible("Amulet_off() was called, but no amulet is worn.");
+		return;
+	}
     switch(uamul->otyp) {
 	case AMULET_OF_ESP:
 		/* need to update ability before calling see_monsters() */