Fixed reading split 7-zip archives.

- TJclSplitStream.Read had a bug that kept it from reading as much data as requested.
- TJclSplitStream.Seek didn't update FVolumePosition when seeking to the next volume failed, which meant seeking to the end and then back to the beginning didn't work.
- TJclSplitStream.InternalLoadVolume's change was just to keep it from calling GetVolumeMaxSize if GetVolume failed.
- TJclCompressionArchive.NeedStreamMaxSize's change supports reading volumes with non-uniform sizes as long as the volumes are added manually.  FVolumeMaxSize is no longer changed when calling OnVolumeMaxSize, since the individual volume's VolumeMaxSize has already been set to that when the volume object was created.

These changes work as long as the calling code uses AddVolume explicitly to add the remaining volumes after creating TJclCompressionArchive, rather than relying on VolumeFileNameMask.  Making it work with VolumeFileNameMask automatically would involve much more extensive changes in JclCompression that could introduce issues with compressing to split volumes, so I've left that for later.