Fixed crash in palette handling when converting certain .png images to .pcx or .bmp.

The existing code expected a palette buffer holding 256 uint32_t's allocated in the data[1] field of the AVFrame structure, but data[1] was NULL. The bug is fixed by using a fixed local array (palette256) to hold the palette instead.

This solves http://ffmpeg.org/trac/ffmpeg/ticket/833

Signed-off-by: Frank Vernaillen <fr_ve@hotmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
