Fix to a sneaky memory leak.
* `hlsl_get_string_buffer()`/`vkd3d_string_buffer_get()` should go with `hlsl_release_string_buffer()`/`vkd3d_string_buffer_release()`. * `vkd3d_string_buffer_init()` should go with `vkd3d_string_buffer_free()`, which only frees the internal `char *` of the string buffer.