Fix #1034 for SET @@sql_mode

[watchpoints]

What changes were proposed in this pull request?

https://github.com/oceanbase/oceanbase/issues/1034

obclient [test]> SET @@sql_mode = X'00FFFFFF';
ERROR 1231 (42000): Variable 'sql_mode' can't be set to the value of 'X00FFFFFF'

• src/share/system_variable/ob_system_variable.cpp
• ret = OB_ERR_WRONG_VALUE_FOR_VAR

Why are the changes needed?

• Expected Behavior

Will break the compatibility? How if so?

NO

Does this PR introduce any user-facing change?

N0

How was this patch tested?

Checklist

• [ ] I've run the tests to see all new and existing tests pass.