In multi-table delete a table for delete can't be used for selecting in
subselects. Appropriate error was raised but wasn't checked which leads to a
crash at the execution phase.

The mysql_execute_command() now checks for errors before executing select
for multi-delete.


mysql-test/t/multi_update.test:
  Added test case for bug#19225: unchecked error results in server crash
mysql-test/r/multi_update.result:
  Added test case for bug#19225: unchecked error results in server crash
sql/sql_parse.cc:
  Fixed bug#19225: unchecked error results in server crash
  The mysql_execute_command() now checks for errors before executing select for multi-delete.