Maintain pointer to end of AVFormatContext.packet_buffer list

Maintain pointer to end of AVFormatContext.packet_buffer list

This changes add_to_pktbuf() to maintain a pointer to the last entry
in the list, avoiding a linear walk-through on each call.  Before this
change, add_to_pktbuf() could take a significant amount of time (10%
of total decoding time), even with input files of several minutes.
After the change, the time spent in this function is barely measurable
with oprofile.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@14654 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b