SET autocommit=1 while XA transaction is active may
cause various side effects, including memory corruption
and server crash.

The problem is that SET autocommit=1 and further queries
attempt to commit local transaction, whereas XA transaction
is still active.

As local and XA transactions are mutually exclusive, this
patch forbids enabling autocommit mode while XA transaction
is active.

mysql-test/r/xa.result:
  A test case for BUG#51342.
mysql-test/t/xa.test:
  A test case for BUG#51342.
sql/set_var.cc:
  Forbid enabling autocommit mode while XA transaction is
  active.