h264: skip chroma edges at the picture boundary while deblocking 4:4:4

This handles macroblock edges for the chroma components in the same way
as for the luma compoment for 4:4:4 streams. The Spec explicitly states
that the deblocking filter is not applied to edges at the boundary of
the picture.

Signed-off-by: Janne Grunau <janne-libav@jannau.net>