pixman-renderer: Don't unreference NULL hw_buffer image

If the buffer for an output was never set, or was reset to NULL at some
point, destroying the renderer output private would cause a crash.

src/compositor.c

@@ -3207,6 +3207,7 @@ usage(int error_code)
 		"  --connector=ID\tBring up only this connector\n"
 		"  --seat=SEAT\t\tThe seat that weston should run on\n"
 		"  --tty=TTY\t\tThe tty to use\n"
+		"  --use-pixman\t\tUse the pixman (CPU) renderer\n"
 		"  --current-mode\tPrefer current KMS mode over EDID preferred mode\n\n");
 
 	fprintf(stderr,

src/pixman-renderer.c

@@ -507,6 +507,8 @@ pixman_renderer_output_destroy(struct weston_output *output)
 	struct pixman_output_state *po = get_output_state(output);
 
 	pixman_image_unref(po->shadow_image);
+
+	if (po->hw_buffer)
 		pixman_image_unref(po->hw_buffer);
 
 	po->shadow_image = NULL;