vampshifting via polymorph

When a vampire that's already in bat or fog form gets polymorphed,
sometimes take on base vampire form instead of always toggling to
fog or bat.

src/mon.c

@@ -4350,7 +4350,11 @@ pickvampshape(struct monst* mon)
         break;
     }
 
-    if (gm.mvitals[mndx].mvflags & G_GENOD)
+    /* return to base form if chosen poly target has been genocided
+       or randomly if already in an alternate form (to prevent always
+       switching back and forth between bat and fog) */
+    if ((gm.mvitals[mndx].mvflags & G_GENOD) != 0
+        || (mon->data != &mons[mon->cham] && !rn2(4)))
         return mon->cham;
 
     return mndx;