gl-renderer: Avoid double-free on init failure

If gl-renderer fails its initialisation, we return to compositor
teardown, which will try to free the renderer if ec->renderer was set.
This is unfortunate when we've already torn it down whilst failing
gl-renderer init, so just clear the renderer member so we don't try to
tear down twice.
Signed-off-by: Daniel Stone <daniels@collabora.com>
Reported-by: Emil Velikov <emil.velikov@collabora.com>

libweston/renderer-gl/gl-renderer.c

@@ -3596,6 +3596,7 @@ fail_terminate:
 	eglTerminate(gr->egl_display);
 fail:
 	free(gr);
+	ec->renderer = NULL;
 	return -1;
 }