Fix pet polyed to long worm staying leashed

Long worms cannot be leashed, but polymorphing a leashed pet
into a long worm kept it leashed.

src/apply.c

@@ -595,7 +595,7 @@ unleash_all()
  *  This ought to exclude various other things, such as lights and gas
  *  spore, is_whirly() critters, ethereal creatures, possibly others.
  */
-static boolean
+boolean
 leashable(mtmp)
 struct monst *mtmp;
 {

src/mon.c

@@ -3762,6 +3762,9 @@ boolean msg;      /* "The oldmon turns into a newmon!" */
     /* take on the new form... */
     set_mon_data(mtmp, mdat);
 
+    if (!leashable(mtmp))
+        m_unleash(mtmp, TRUE);
+
     if (emits_light(olddata) != emits_light(mtmp->data)) {
         /* used to give light, now doesn't, or vice versa,
            or light's range has changed */