A bogus error message was issued for any outer references occurred in
ON expressions used in subqueries. This prevented execution of queries
containing subqueries as soon as they used outer references in their ON
clauses. This happened because the Name_resolution_context structure
created for any ON expression erroneously had the field outer_context set
to NULL. The fields select_lex of this structure was not set correctly
either.

The idea of the fix was taken from mysql code of the function
push_new_name_resolution_context().

Approved by dmitry.shulga@mariadb.com