add sym_validate_len() to validate symbol length

This also fixes an off-by-one in mrb_check_intern().
use RITE_LV_NULL_MARK for better readability

The comment didn't say why the length is reserved.
Using RITE_LV_NULL_MARK makes things a bit clearer.
Merge pull request #2576 from cremno/correctly-validate-symbol-length

correctly validate symbol length