cogl/renderer: Initialize variable

Otherwise gcc cannot detect that the variable will be
assigned before use.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4349>

Author: fmuellner

cogl/cogl/cogl-renderer.c

@@ -265,11 +265,13 @@ _cogl_renderer_choose_driver (CoglRenderer *renderer,
                               GError **error)
 {
   const char *driver_name = g_getenv ("COGL_DRIVER");
-  CoglDriverId picked_driver, driver_override = COGL_DRIVER_ID_ANY;
+  CoglDriverId picked_driver, driver_override;
   const char *invalid_override = NULL;
   const char *libgl_name = NULL;
   int i;
 
+  picked_driver = driver_override = COGL_DRIVER_ID_ANY;
+
   if (driver_name)
     {
       driver_override = driver_name_to_id (driver_name);