player: fix exiting if both audio and video fail initializing

The player was supposed to exit playback if both video and audio failed
to initialize (or if one of the streams was not selected when the other
stream failed). This didn't work; for one this check was missing from
one of the failure paths. And more importantly, both checked the
current_track array incorrectly.

Fix these issues, and move the failure handling code into a common
function.

CC: @mpv-player/stable

Conflicts:
	player/audio.c
	player/video.c
Release 0.6.2