Fixed a bug in IsoMediaHeaderBox.to_media_time_from() and from_media_time_to()

Due to incorrectly-placed parens, mediatime values larger than uint32.MAX
 were being converted to 0.

Also added logic in IsoSampleTableBox.access_point_sample_for_byte_offset
 to set the AccessPoint.time_offset value to 0 if the sample_number is 0.

Tested with open range values on B-AVC_MP4_MP_SD-08.MP4, which was causing
 an invalid access violation due to the open range end being uint64.MAX.