SET STATEMENT.

Server built with debug asserts, without debug crashes if a user tries
to run a stored procedure that constains query with subquery that include
either LIMIT or LIMIT OFFSET clauses.

The problem was that Item::fix_fields() was not called for the items
representing LIMIT or OFFSET clauses.

The solution is to call Item::fix_fields() right before evaluation in
st_select_lex_unit::set_limit().

mysql-test/r/sp.result:
  Added testcase result for bug#12621017. Updated testcase result for
  bug 11918.
mysql-test/t/sp.test:
  Added testcase for bug#12621017. Addressed review comments for Bug 11918
  (added tests for use LIMIT at stored function).
sql/item.h:
  Addressed review comments for Bug 11918.
sql/share/errmsg-utf8.txt:
  Addressed review comments for Bug 11918.
sql/sp_head.cc:
  Addressed review comments for Bug 11918.
sql/sql_lex.cc:
  Added call fix_fields() for item just before its evaluation.
sql/sql_yacc.yy:
  Addressed review comments for Bug 11918.