Fix: Juiblex could be summoned an infinite number of times

This is due to an interaction with the code that prevents him from being
made extinct (if he was, he couldn't split). Fix this by changing the
check for a summoned demon lord to respect its born counter rather than
its extinction status (should be the same thing as long as this only
considers uniques).

src/minion.c

@@ -609,7 +609,12 @@ dlord(aligntyp atyp)
 
     for (tryct = !In_endgame(&u.uz) ? 20 : 0; tryct > 0; --tryct) {
         pm = rn1(PM_YEENOGHU + 1 - PM_JUIBLEX, PM_JUIBLEX);
-        if (!(gm.mvitals[pm].mvflags & G_GONE)
+        /* This previously checked G_GONE (as dprince still does), but Juiblex
+         * is weird in that he splits, and in order to split he doesn't get
+         * G_EXTINCT set. Instead, check the born counter (which also works fine
+         * for Yeenoghu or any other archfiend since the outcome is the same -
+         * if they have already been generated, they can't be summoned. */
+        if (!(gm.mvitals[pm].born)
             && (atyp == A_NONE || sgn(mons[pm].maligntyp) == sgn(atyp)))
             return pm;
     }