montraits of weapon-wielding monsters

Have save_mtraits() clear wielded weapon when attaching monster
attributes to a corpse object.

And have monster sanity check verify that wielded weapon is in the
monster's inventory.

src/mkobj.c

@@ -2154,6 +2154,7 @@ save_mtraits(struct obj *obj, struct monst *mtmp)
         mtmp2->nmon = (struct monst *) 0;
         mtmp2->data = (struct permonst *) 0;
         mtmp2->minvent = (struct obj *) 0;
+        MON_NOWEP(mtmp2); /* mtmp2->mw = (struct obj *) 0; */
         if (mtmp->mextra)
             copy_mextra(mtmp2, mtmp);
         /* if mtmp is a long worm with segments, its saved traits will

src/mon.c

@@ -233,6 +233,19 @@ sanity_check_single_mon(
                        mtmp->m_id, distu(mtmp->mx, mtmp->my));
 #endif
     }
+    if (MON_WEP(mtmp)) { /* mtmp->mw */
+        struct obj *o;
+
+        for (o = mtmp->minvent; o; o = o->nobj)
+            if (o == MON_WEP(mtmp))
+                break;
+        if (!o) {
+            o = MON_WEP(mtmp);
+            impossible("monst (%s: %u) wielding %s (%u) not in inventory",
+                       pmname(mtmp->data, Mgender(mtmp)), mtmp->m_id,
+                       safe_typename(o->otyp), o->o_id);
+        }
+    }
 }
 
 void