The Diagnostic_area caused an assertion failure in debug mode when
the disk was full.
  
By setting the internal error handler to ignore errors caused by
underlying logging methods, the error is avoided.


sql/log.cc:
  MYSQL_QUERY_LOG::write uses IOCACHE for writing data. If writing fails 
  an error is pushed by the procedure my_error which in turn invokes
  the default error handler my_message_sql. When the error is set the
  Diagnostic_area status changes and later trigger an assertion on the
  next consecutive status change which happens by for example the 
  function my_ok(). By intercepting the error handler from the logger
  interface this error can be silenced.