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/do_name.c

@@ -1107,6 +1107,9 @@ christen_monst(struct monst *mtmp, const char *name)
     new_mgivenname(mtmp, lth); /* removes old name if one is present */
     if (lth)
         Strcpy(MGIVENNAME(mtmp), name);
+    /* if 'mtmp' is leashed, persistent inventory window needs updating */
+    if (mtmp->mleashed)
+        update_inventory(); /* x - leash (attached to Fido) */
     return mtmp;
 }