iproto: don't account message twice in case of override fallback

We need to call `tx_accept_msg` in `tx_process_override` before we pass
message to the override handler. Unfortunately if handler response with
IPROTO_HANDLER_FALLBACK we call the builtin handler for message that
calls `tx_accept_msg` again which is not expected. Some actions of
this function are idempotent and some are not.

Let's make the function NOP if it called once again.

Closes #9345

NO_DOC=bugfix