comctl32/toolbar: When a null lParam is passed to TB_GETBUTTONTEXTA, we should not return -1 but the string size.

It constains the register name plus potential modifiers, so it should be
larger than reg_name instead of smaller.

The version tokens in SM1-3 and SM4 are similar, but not the same.

This makes the location of used program.local parameters more
predictable and eases sharing this space with other private constants.