DiscIO/DirectoryBlob: Handle reads between files.

[AdmiralCurtiss]

If someone passes an offset past the end of a file but before the start of another file to DiscContentContainer::Read(), the PadToAddress() call completely fills the buffer with zeroes and reduces the length to 0, but the code flow then tries to call it->Read() anyway, triggering a debug assert about an out-of-bounds offset.

To easily observe this just launch an extracted game in a Debug build, the read from VolumeDisc::IsNKit() is between files in our built disc layout.