Improved handling of situations when we encounter error during
CREATE PROCEDURE (FUNCTION/TRIGGER/...) and bail out of yyparse()
without restoring proper THD::lex.


sql/sp.cc:
  We do not need to call sp_head::restore_lex() explicitly to restore right
  value of THD::lex in case when we have encountered error during parsing.
  Now we do this in sp_head::~sp_head() instead.
sql/sp_head.cc:
  sp_head::destroy():
   Fixed cleaning up of stack of auxilary LEXes. 
   We also restore right value of THD::lex during this process now.
sql/sql_parse.cc:
  We do not need to call sp_head::restore_lex() explicitly to restore right
  value of THD::lex in case when we have encountered error during parsing.
  Now we do this in sp_head::~sp_head() instead.
sql/sql_prepare.cc:
  We do not need to call sp_head::restore_lex() explicitly to restore right
  value of THD::lex in case when we have encountered error during parsing.
  Now we do this in sp_head::~sp_head() instead.
sql/sql_trigger.cc:
  We do not need to call sp_head::restore_lex() explicitly to restore right
  value of THD::lex in case when we have encountered error during parsing.
  Now we do this in sp_head::~sp_head() instead.