Fixes avpicture_layout to not write past buffer end.

avpicture_get_size() returns the size of buffer required for avpicture_layout.
For pseudo-paletted formats (gray8...) this size does not include the palette.
However, avpicture_layout doesn't know this and still writes the palette. Consequently,
avpicture_layout writes passed the length of the buffer. This fixes it
by fixing avpicture_layout so that it doesn't write the palette for these formats.

Signed-off-by: Matthew Einhorn <moiein2000@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>