ha_innobase::index_read(): If an autocommit non-locking transaction was
already started, refuse to access a SPATIAL INDEX.
Once a non-locking autocommit transaction has started, it must remain
in that mode (not acquire any locks).

This should fix one cause of the assertion failure that would occur in
DeadlockChecker::check_and_resolve() under heavy load, presumably
due to concurrent execution of trx_commit_in_memory().