tarantool / tarantool / 5132905768

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replication: fix crash on access to a not yet ready relay

All the code outside of relay.cc judges about relay's liveliness looking
only at relay state. When relay->state is RELAY_FOLLOW, the relay is
considered operational.

This is not always true: for example, both relay_push_raft() and
relay_trigger_vclock_sync() are only possible after relay thread pairs
with tx via the cbus. This happens **after** the relay enters
RELAY_FOLLOW state.

Fix the possible access to uninitialized cpipe by
relay_trigger_vclock_sync(): make it a nop until the relay is paired
with tx.

Closes #7991

NO_DOC=bugfix
NO_TEST=covered by replication-luatest/linearizable_test.lua

(cherry picked from commit 0ef5e3b22)

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