xcb: Fix _put_shm_image_boxes if no SHM available

This function always returned CAIRO_INT_STATUS_SUCCESS, even if it didn't do
anything. This commit makes the function return CAIRO_INT_STATUS_UNSUPPORTED
instead.

No unit test for this, because we currently do not test the !SHM case
automatically. Perhaps we should?

Signed-off-by: Uli Schlachter <psychon@znc.in>

src/cairo-xcb-surface.c

@@ -646,9 +646,8 @@ _put_shm_image_boxes (cairo_xcb_surface_t    *surface,
 						     shm_info->offset);
 	    }
 	}
+	return CAIRO_INT_STATUS_SUCCESS;
     }
-
-    return CAIRO_INT_STATUS_SUCCESS;
 #endif
 
     return CAIRO_INT_STATUS_UNSUPPORTED;