fix #K3496 - leash description when pet changes

If persistent inventory is displayed and contains an entry for a leash
attached to a pet and the pet's type or name changes, the perm_invent
window didn't get updated to reflect the new leash information:
 x - leash (attached to <mon>)
Report was for polymorph but applied to growing into bigger form and
to being (re-/un-)christened as well.

src/makemon.c

@@ -1914,6 +1914,9 @@ grow_up(struct monst *mtmp, struct monst *victim)
         lev_limit = (int) mtmp->m_lev; /* never undo increment */
 
         mtmp->female = fem; /* gender might be changing */
+        /* if 'mtmp' is leashed, persistent inventory window needs updating */
+        if (mtmp->mleashed)
+            update_inventory(); /* x - leash (attached to a <mon> */
     }
 
     /* sanity checks */