Add check for negative file length.
This prevents a crash when querying the length of a PVR live stream from Python.
PVR addons can return -1 for LengthLiveStream() which originally resulted in a
request for 4294967295 bytes which failed.
Merge pull request #4434 from margro/gotham_vfs_crash

[Gotham] Crash fix: Add check for negative file length.