The problem was that if UPDATE with subselect caused a
deadlock inside InnoDB, this deadlock was not properly
handled by the SQL layer. This meant that the SQL layer
would try to unlock the row after InnoDB had rolled
back the transaction. This caused an assertion inside
InnoDB.
  
This patch fixes the problem by checking for errors
reported by SQL_SELECT::skip_record() and not calling
unlock_row() if any errors have been reported.

This bug is similar to Bug#13586591, but for UPDATE
rather than DELETE. Similar issues in filesort/opt_range/
sql_select will be investigated and handled in the scope
of Bug#16767929