This ensure get_buffer() won't be confused with an AVFrame pointer
which looks like it wasn't released.
Originally committed as revision 8124 to svn://svn.ffmpeg.org/ffmpeg/trunk
libavutil/common.h for platforms with no inttypes.h.
patch by Steve Lhomme, slhomme divxcorp com
Originally committed as revision 6790 to svn://svn.ffmpeg.org/ffmpeg/trunk