Set restrictions on packet_size, as per ISO-11172 / H-222 specifications

(max packet size should fit in 13 bits as a kB value, so 1<<23, plus the
header which is 10 bytes), and as per mpegenc.c internal assumptions
(min packet size is 20 bytes). See "[PATCH] make packet_size in
AVFormatContext unsigned" thread.

Originally committed as revision 19215 to svn://svn.ffmpeg.org/ffmpeg/trunk