gl-renderer: Drop unnecessary NULL check

All the callers of flush_damage guarantee we'll have a buffer.

Signed-off-by: Daniel Stone <daniels@collabora.com>

libweston/renderer-gl/gl-renderer.c

@@ -1844,12 +1844,11 @@ gl_renderer_flush_damage(struct weston_surface *surface,
 	uint8_t *data;
 	int i, j, n;
 
+	assert(buffer);
+
 	pixman_region32_union(&gs->texture_damage,
 			      &gs->texture_damage, &surface->damage);
 
-	if (!buffer)
-		return;
-
 	/* Avoid upload, if the texture won't be used this time.
 	 * We still accumulate the damage in texture_damage, and
 	 * hold the reference to the buffer, in case the surface