Before MDEV-14638, there was no race condition between the
execution of fetch_data_into_cache() and transaction commit.

fetch_data_into_cache(): Acquire trx_t::mutex before checking
trx_t::state, to prevent a concurrent transition from
TRX_STATE_COMMITTED_IN_MEMORY to TRX_STATE_NOT_STARTED
in trx_commit_in_memory().