CLX-103: xpand.update fails after for CLX-55, part #2

Fix the complaint from safemalloc about leaked 8 bytes.

When mysql_update() uses direct update, it will still create a quick select
(doesn't make much sense, does it?)
Quick select will be initialized, which will call ha_xpand::index_init
which will allocate ha_xpand::scan_fields.

Then, ha_xpand::index_end will be called but it will fail to free scan_fields.

storage/xpand/ha_xpand.cc

@@ -875,7 +875,10 @@ int ha_xpand::index_end()
   if (scan_cur)
     DBUG_RETURN(rnd_end());
   else
+  {
+    my_bitmap_free(&scan_fields);
     DBUG_RETURN(0);
+  }
 }
 
 int ha_xpand::rnd_init(bool scan)