mplayer: don't show bogus audio PTS during hr-seek

This fixes a problem that happened with syncplay.pl [1] when ad_mpg123
was in use, and get_current_time() returning a bogus time position.
This only happens during seeking; the reported time is correct after the
seek is done.

The audio PTS as returned by playing_audio_pts() is simply bogus during
hr-seek. With ad_ffmpeg, it was actually set to MP_NOPTS_VALUE during
seeking, so get_current_time() did a fallback to the video PTS. However,
ad_mpg123 is different and explicitly decodes some audio when resetting
on seek (reasons why it does this unknown and uninvestigated; apparently
it's to reinit libmpg123). As a result, the audio PTS was set to the
start position of the seek (or something similar), which could be very
different from the seek target time.

This confused syncplay. It got the bogus time because it spams the
player with read commands to the "time-pos" property, so this corner
case was hit.

Fix this by making get_current_time() return the seek target time if
hr-seek is active. This should make behavior the same as before commit
3f949cf "mplayer: prefer audio PTS over video PTS for status line".

[1] http://syncplay.pl