Fix bug #13180 thd->allow_sum_funcs wasn't reset before query processing.

thd->allow_sum_func was left 'true' after previous statement thus allowing
sum funcs to be present in conditions.

thd->allow_sum_func should be set to 0 for each query and each prepared
statement reinitialization. This is done in lex_start() and 
reset_stmt_for_execute().

mysql-test/r/update.result

@@ -251,3 +251,15 @@ f1	f2
 1	1
 2	2
 drop table t1,t2;
+create table t1(f1 int);
+select DATABASE();
+DATABASE()
+test
+update t1 set f1=1 where count(*)=1;
+ERROR HY000: Invalid use of group function
+select DATABASE();
+DATABASE()
+test
+delete from t1 where count(*)=1;
+ERROR HY000: Invalid use of group function
+drop table t1;

mysql-test/t/update.test

@@ -215,4 +215,16 @@ UPDATE t1 SET t1.f2=(SELECT MAX(t2.f4) FROM t2 WHERE t2.f3=t1.f1);
 select * from t1;
 drop table t1,t2;
 
+#
+# Bug #13180 sometimes server accepts sum func in update/delete where condition
+#
+create table t1(f1 int);
+select DATABASE();
+--error 1111
+update t1 set f1=1 where count(*)=1;
+select DATABASE();
+--error 1111
+delete from t1 where count(*)=1;
+drop table t1;
+
 # End of 4.1 tests

sql/sql_lex.cc

@@ -160,6 +160,7 @@ void lex_start(THD *thd, uchar *buf,uint length)
   lex->duplicates= DUP_ERROR;
   lex->ignore= 0;
   lex->proc_list.first= 0;
+  thd->allow_sum_func= 0;
 }
 
 void lex_end(LEX *lex)

sql/sql_prepare.cc

@@ -1738,6 +1738,7 @@ static void reset_stmt_for_execute(Prepared_statement *stmt)
   lex->current_select= &lex->select_lex;
   if (lex->result)
     lex->result->cleanup();
+  thd->allow_sum_func= 0;
 }