asfdec: break if EOF is reached after asf_read_packet_header

asf_read_payload can unset eof_reached, so check it also before calling
that function.

This fixes infinite loops.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Alexandra Hájková <alexandra@khirnov.net>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
