Bug#31588: buffer overrun when setting variables

post-merge fixes: format string was changed 5.0 -> 5.1, resulting in longer output

mysql-test/r/variables.result

@@ -798,7 +798,7 @@ select @@query_prealloc_size = @test;
 @@query_prealloc_size = @test
 1
 set global sql_mode=repeat('a',80);
-ERROR 42000: Variable 'sql_mode' can't be set to the value of 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
+ERROR 42000: Variable 'sql_mode' can't be set to the value of 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
 End of 4.1 tests
 create table t1 (a int);
 select a into @x from t1;