avformat/dashdec: Cosmetics

avformat/dashdec: Cosmetics

1. Perform the necessary reindentations after the last few commits.
2. Adapt switches to the ordinary indentation style.
3. Now that the effective lifetimes of the variables containing
the freshly allocated strings used when parsing the representation
are disjoint, the variables can be replaced by a single variable.
Doing so has the advantage of making it more clear that these are
throwaway variables, hence it has been done.

Reviewed-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>