PlatformAddress.check returns true with the invalid string

[sgkim126]

Because the logic of PlatformAddress.check is different from PlatformAddress.fromString, check returns true for the invalid string.
For example, cccqgfrg4ncjqfrg4ncjqfrg4ncjqfrg4ncjq2tpqzg is not a valid platform address because the version is 2, but check returns true.