R926 - polymorphed player does not explode at thin air

Reported that player polymorphed into an exploding monster does not explode
when attacking thin air.  Also noticed that if player is Unchanging, they
wouldn't die after exploding in the existing code.  Set u.mh = -1 to ensure
they won't still be around after exploding.

src/uhitm.c

@@ -1920,8 +1920,10 @@ use_weapon:
 			impossible("strange attack of yours (%d)",
 				 mattk->aatyp);
 	    }
-	    if (dhit == -1)
+	    if (dhit == -1) {
+		u.mh = -1;	/* dead in the current form */
 		rehumanize();
+	    }
 	    if (sum[i] == 2)
 		return((boolean)passive(mon, 1, 0, mattk->aatyp));
 							/* defender dead */