Fix BUG#2674: Crash if return system variable in stored procedures.

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.

mysql-test/r/sp.result

@@ -989,6 +989,12 @@ call bug2614()|
 call bug2614()|
 drop table t3|
 drop procedure bug2614|
+create function bug2674 () returns int
+return @@sort_buffer_size|
+select bug2674()|
+bug2674()
+262136
+drop function bug2674|
 drop table if exists fac|
 create table fac (n int unsigned not null primary key, f bigint unsigned)|
 create procedure ifac(n int unsigned)