If frame count is known at init time (cli & vfw), then abort if the 2nd pass

If frame count is known at init time (cli & vfw), then abort if the 2nd pass
exceeds the length of the 1st pass.
If it's not known (mencoder), then report a non-fatal error when we run off the
end of the 1st pass stats, and switch to constant QP.

git-svn-id: svn://svn.videolan.org/x264/trunk@340 df754926-b1dd-0310-bc7b-ec298dee348c