ohitmon: obfree vs delobj

Make missiles that aren't launched by the hero and that hit a monster
use the routine that protects the Amulet and invocation items against
being deleted.  I don't think there are any cases where this matters
because those items don't break when they hit something, but be more
cautious.

src/mthrowu.c

@@ -1,4 +1,4 @@
-/* NetHack 3.7	mthrowu.c	$NHDT-Date: 1620923922 2021/05/13 16:38:42 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.114 $ */
+/* NetHack 3.7	mthrowu.c	$NHDT-Date: 1629497158 2021/08/20 22:05:58 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.117 $ */
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /*-Copyright (c) Pasi Kallinen, 2016. */
 /* NetHack may be freely redistributed.  See license for details. */
@@ -154,7 +154,7 @@ drop_throw(
             }
         }
     } else
-        obfree(obj, (struct obj *) 0);
+        delobj(obj);
     return retvalu;
 }