avformat/avidec: ensure that palette does not contain the BottomUp info.

Considering the palette is located at the end of extradata may be flawed
when the extradata contains the palette followed by the BottomUp field.
When the BottomUp field is present, exclude it from the palette.
Fixes part of ticket #1304

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>