resulted in a wrong error message.

The nest_level counter indicates the depth of nesting for a subselect. It is
needed to properly resolve aggregate functions in nested subselects. Obviously 
it shouldn't be incremented for UNION parts because they have the same level of
nesting. This counter was incremented by 1 in the mysql_new_select() function
for any new select and wasn't decremented for UNION parts. This resulted in
wrongly reported error messages.

Now the nest_level counter is decremented by 1 for any union part.


mysql-test/t/union.test:
  Added test case for the bug#18175: The nest_level counter wasn't decremented for union parts which
  resulted in a wrong error message.
mysql-test/r/union.result:
  Added test case for the bug#18175: The nest_level counter wasn't decremented for union parts which
  resulted in a wrong error message.
sql/sql_yacc.yy:
  Fixed bug#18175: The nest_level counter wasn't decremented for union parts which
  resulted in a wrong error message.
  Now the nest_level counter is decremented by 1 for any union part.