movenc: Remove an outdated comment

QuickTime does support files with an empty initial movie
these days.

Signed-off-by: Martin Storsjö <martin@martin.st>
movenc: Avoid leaking locally allocated data when returning on errors

Signed-off-by: Martin Storsjö <martin@martin.st>
movenc: Require samples before trying to write edts

This avoids a potential crash if writing a fragmented psp mp4
(which probably is only a hypothetical scenario).

Signed-off-by: Martin Storsjö <martin@martin.st>