Up until now, read_frame_internal always initialized the packet it
received. But since the recent changes to ff_read_packet, this is no
longer needed: If the parsing queue is initially empty upon entering
read_frame_internal, the packet will now either contain content upon
success or be blank upon failure of ff_read_packet. If the parsing
queue is initially not empty, the packet will be overwritten with the
oldest one from the parsing queue.

Similarly, it is unnecessary to initialize ret in read_frame_internal.

In parse_packet, it is easily possible to only initialize the packet
used as temporary storage for the output if said packet is used at all;
furthermore, this packet doesn't need to be zero-initialized, because
av_init_packet will initialize every field except size and data and
those fields will be set by av_parser_parse2.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>