playlist: Fix crasher with playlists using reserved columns.

I introduced this bug in commit a800c1b3ffeb trying to fix a different
memory mis-using bug. Sigh.

In this case playlists that reserve additional columns would get a valid
memory assignment but would be copied into the memory block incorrectly.
This needed a back_inserter as well.

Thanks to Markus for pointing out the regression, then opening a bug,
then leaving a comment to point out both of these, and then drafting a
patch that fixes the bug.

I can confirm it works in my testing, it also seems to address a bug
relating to playback stopping abrupting at the end of a track instead of
skipping to the next track, but who knows at this point.

BUG:420718
FIXED-IN:20.04.1