The 0.5 ffmpeg branch includes libswscale directly, not as an external,
breaking the dist (and warning in autogen.sh) about not being able to
update its revision.

Add some code to not update the svn external if the
FFMPEG_EXTERNALS_REVISION variable is empty.

Also, in autogen.sh check that the current FFmpeg checkout is from the
right branch according to ffmpegrev, and check it out again if
it's not.