Avoid appending an empty slot to an user data array when user_data is NULL.

Otherwise, calling cairo_set_user_data(cr, key, 0, 0) many times
causes a long user data array, almost all of whose slots are empty.
It leads to unnecessarily much memory consumption and long execution time of
cairo_set_user_data(cr, key, 0, 0) and cairo_get_user_data(cr, key) after
it.

This issue probably happens since the commit
http://cgit.freedesktop.org/cairo/commit/?id=9341c254a

Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

src/cairo-array.c

@@ -485,6 +485,9 @@ _cairo_user_data_array_set_data (cairo_user_data_array_t     *array,
 	return CAIRO_STATUS_SUCCESS;
     }
 
+    if (user_data == NULL)
+	return CAIRO_STATUS_SUCCESS;
+
     status = _cairo_array_append (array, &new_slot);
     if (unlikely (status))
 	return status;