libtxproto: Add free'd callback on tx_io_register_cb

[rofferom]

This will allow to free stuff inside the userdata.

[cyanreg]

Is there a specific need for this? Userdata is meant to be some user context that users free themselves, as it is created before registering the callback.

[rofferom]

Actually This will allow to free stuff inside the userdata. is a consequence of the patch. At first I wanted to be notified of the AVBufferRef * that is returned. It also allow to use the free callback to understand when the userdata isn't needed anymore.