failed"

Do not assume that SQL prepared statements always run in text protocol.
When invoked from a stored procedure, which is itself invoked
by means of prepared CALL statement, the protocol may be binary.
Juggle with the protocol only when we want to change it
to binary in COM_STMT_EXECUTE, COM_STMT_PREPARE.

This is a backport from 5.4/6.0, where the bug was fixed
as part of WL#4264 "Backup: Stabilize Service Interface"