Live streams have sometimes no valid duration, so setting the
`_positionSet` state only in case of `_mediaDuration > 0` leads to
an issue that seeking in such streams only works for the first time,
as `_positionSet` is initialized as NO, then it is set to YES in
`_setPositionForReal` and would be never set back to NO, causing all
future seeks to be ignored.

Fix videolan/VLCKit#157