Since the recent changes to ff_packet_list_put, the source packet will
be automatically reset when the reference is moved to the packet list,
so that it is unnecessary to reinitialize the packet in the loops in
parse_packet and ff_read_packet; initializing once at the beginning is
enough.

This also fixes a potential, but currently unexisting problem: If the
raw packet buffer was initially not empty and probe_codec() failed,
then the packet returned would not be initialized. But given that
probe_codec() currently can't fail (always returns 0) this was not an
acute danger.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>