Fix for MXS-968

This commit adds a free() to null_auth_free_client_data, which plugs
the memory leak in maxinfo.

Also, this commit fixes some segfaults when multiple threads are
running status_row() or variable_row(). The functions use
statically allocated index variables, which often go out-of-bounds
in concurrent use. This fix changes the indexes to thread-specific
variables, with allocating and deallocating. This does seem to slow
the functions down somewhat.

server/modules/authenticator/null_auth.c

@@ -150,4 +150,8 @@ null_auth_is_client_ssl_capable(DCB *dcb)
  * @param dcb Request handler DCB connected to the client
  */
 static void
-null_auth_free_client_data(DCB *dcb) {}
+null_auth_free_client_data(DCB *dcb)
+{
+    free(dcb->data);
+    dcb->data = NULL;
+}