Fix: artifacts silently removed from the game

Death drops will generate a random item, which may be an artifact.
After the object is created, some rules are applied to check whether its
size matches the monster that dropped it; if it fails these checks,
the item is deleted.  aosdict recently pointed out that if the death
drop was an artifact, this would permanently remove it from the game.

Use artifact_exist(..., FALSE) in various cases where potential
artifact objects are created, then immediately removed.  This will
prevent artifacts from being removed from play by marking the artifact
as available to create again.

src/mon.c

@@ -2954,6 +2954,8 @@ xkilled(
             if (mdat->msize < MZ_HUMAN && otyp != FIGURINE
                 /* oc_big is also oc_bimanual and oc_bulky */
                 && (otmp->owt > 30 || objects[otyp].oc_big)) {
+                if (otmp->oartifact)
+                    artifact_exists(otmp, safe_oname(otmp), FALSE);
                 delobj(otmp);
             } else if (!flooreffects(otmp, x, y, nomsg ? "" : "fall")) {
                 place_object(otmp, x, y);

src/sp_lev.c

@@ -2219,6 +2219,8 @@ create_object(object* o, struct mkroom* croom)
                 cobj->owt = weight(cobj);
             } else {
                 obj_extract_self(otmp);
+                if (otmp->oartifact)
+                    artifact_exists(otmp, safe_oname(otmp), FALSE);
                 obfree(otmp, NULL);
                 return;
             }