Had to initialize some fields in lex before parsing an SP read from
mysql.proc.


mysql-test/r/sp.result:
  New test case for BUG#2674.
mysql-test/t/sp.test:
  New test case for BUG#2674.
sql/sp.cc:
  We need to initialize some thd->lex fields (e.g. unit master pointers) since
  LEX::uncacheable() depends on this, and it's called when looking up system
  variables during parsing.