dashdec: Only free url string if being reused

If no representation bandwidth value is set, the url value returned
by get_content_url is corrupt (as it has been freed).
This change ensures the url string is not freed unless it is about
to be reused
Changes since v1:
 1 removed the unneeded 'if' statement (as pointed out by Michael Niedermayer
 2 added comment to make it clear why the av_free was required
Signed-off-by: Brendan McGrath <redmcg@redmandi.dyndns.org>
