Incorrect value returned for DEMUX_GET_POSITION by TS muxer DTS is not available

When playing a TS muxed stream for which DTS is not available (e.g. playing video from a TS file accessed via HTTP), the time position should be calculated from the current byte position, but by mistake a wrong estimation is made from an invalid DTS value and the result produced for DEMUX_GET_POSITION is incorrect.

The issue was introduiced in commit 178c1a4d which modified modules/demux/mpeg/ts.c line 930 (nowadays 929), changing ...

p_pmt->i_last_dts > VLC_TS_INVALID

p_pmt->i_last_dts != VLC_TS_INVALID

This change did not 'fix' the comparison but broke it, because the field p_pmt->i_last_dts is initialized (somewhere else) with value -1, and VLC_TS_INVALID (nowadays VLC_TICK_INVALID) equals 0. As a consequence, the value -1 is used as a parameter for calling TimeStampWrapAround, what causes that DEMUX_GET_POSITION returns a completely wrong value.

The effect of this error is that, for that particular kind of stream, the VLC time slider does not show the current play position.

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