Reset thd->lex->current_select for SP

current_select may point to data from old parser states
when calling a stored procedure with CALL

The failure happens in Item::Item when testing if we are
in having.

Fixed by explicitely reseting current_select in do_execute_sp()
and in sp_rcontext::create(). The later is also needed for
stored functions().

sql/sql_parse.cc

@@ -2876,6 +2876,12 @@ static bool do_execute_sp(THD *thd, sp_head *sp)
   ha_rows select_limit= thd->variables.select_limit;
   thd->variables.select_limit= HA_POS_ERROR;
 
+  /*
+    Reset current_select as it may point to random data as a
+    result of previous parsing.
+  */
+  thd->lex->current_select= NULL;
+
   /*
     We never write CALL statements into binlog:
      - If the mode is non-prelocked, each statement will be logged