Fix panic on thrown potion of acid when swallowed

When you're swallowed and throw a missile that kills the swallower,
the thrown missile is picked up by the swallower before death.
This caused a panic when you threw a potion, which was first picked up
by the swallower, which upon death dropped the potion on the floor,
but then throw code was trying to destroy the potion ...

src/mon.c

@@ -2989,6 +2989,8 @@ xkilled(
         EDOG(mtmp)->killed_by_u = 1;
 
     if (wasinside && g.thrownobj && g.thrownobj != uball
+        /* don't give to mon if missile is going to be destroyed */
+        && g.thrownobj->oclass != POTION_CLASS
         /* don't give to mon if missile is going to return to hero */
         && g.thrownobj != (struct obj *) iflags.returning_missile) {
         /* thrown object has killed hero's engulfer; add it to mon's