using @@var_name in proc
  Made sure we don't lose the tokenizer state when swapping lex in during SET
  parsing.


mysql-test/r/sp.result:
  New test case for BUG#9538.
mysql-test/t/sp.test:
  New test case for BUG#9538.
sql/sp_head.cc:
  Keep lex->next_state when swapping lex during SP parsing (for SET mainly),
  otherwise the tokenizer gets confused.