stream: make `GetPtsDelay` impossible to fail

The documentation and code-usage state that streams cannot have that control fail, only some types of demuxes can.

From the demuxer documentation:

/** Retrieves the PTS delay (roughly the default buffer duration).

• Can fail only if synchronous and not an access-demuxer. The
• underlying input stream then determines the PTS delay.
• arg1= vlc_tick_t * */ DEMUX_GET_PTS_DELAY = 0x101,

From the stream documentation:

STREAM_GET_PTS_DELAY = 0x101,/*< arg1= vlc_tick_t res=cannot fail */

This patch reflects this behavior in the stream API.