asfdemux: Always re-initialize metadata and global_metadata

When transitioning from the PAUSED state, to the READY state, and back,
metadata and global_metadata are left uninitialized, unlike when the
demxuer transitions from NULL to READY, then to PAUSED.  I have found
this to cause a segmentation fault when fields in these structures are
set.
25 jobs for !43 with reset-fix in 25 minutes and 8 seconds (queued for 3 seconds)
merge request