partition is corrupt

Post push fix

an DBUG_ASSERT broke the embedded server, fixed by initializing
it in the embedded version of Protocol_text::prepare_for_resend

libmysqld/lib_sql.cc:
  Bug#20129: ALTER TABLE ... REPAIR PARTITION ... complains that
  partition is corrupt
  
  Post push fix
  
  an DBUG_ASSERT in Protocol_text::store broke the embedded
  server, fixed by initializing it in the embedded version
  of Protocol_text::prepare_for_resend