Set xbzrle buffers to NULL after freeing them to avoid double free errors

Signed-off-by: Orit Wasserman <owasserm@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

arch_init.c

@@ -617,6 +617,9 @@ static void migration_end(void)
         g_free(XBZRLE.current_buf);
         g_free(XBZRLE.decoded_buf);
         XBZRLE.cache = NULL;
+        XBZRLE.encoded_buf = NULL;
+        XBZRLE.current_buf = NULL;
+        XBZRLE.decoded_buf = NULL;
     }
 }