Fix a minor memory leak

When xcb_request_check() returns an error, something has to clean up and
free this error.

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

libi3/draw_util.c

@@ -47,6 +47,7 @@ void draw_util_surface_init(xcb_connection_t *conn, surface_t *surface, xcb_draw
     xcb_generic_error_t *error = xcb_request_check(conn, gc_cookie);
     if (error != NULL) {
         ELOG("Could not create graphical context. Error code: %d. Please report this bug.\n", error->error_code);
+        free(error);
     }
 
     surface->surface = cairo_xcb_surface_create(conn, surface->id, visual, width, height);