read AVI palette from the end of extradata

Official AVI specification says that stream header in case of video contains
BITMAPINFO, which is equal to BITMAPINFOHEADER and optional palette. Currently
lavf AVI demuxer thinks otherwise which produces garbage on codecs that have
both palette and extradata (luckily, there are not so many such codecs).

An example of such file is:
http://samples.multimedia.cx/V-codecs/KMVC/baseball1.avi
(IIRC, MSS1 or MSS2 also had such situation but they are still not supported
by lavc).

As a side note, passing palette in extradata as it's been done previously is
not quite correct since proper _extra_ data is surplus bytes in
BITMAPINFOHEADER, not including palette.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavformat/avidec.c

@@ -590,12 +590,16 @@ static int avi_read_header(AVFormatContext *s, AVFormatParameters *ap)
                     /* This code assumes that extradata contains only palette. */
                     /* This is true for all paletted codecs implemented in Libav. */
                     if (st->codec->extradata_size && (st->codec->bits_per_coded_sample <= 8)) {
+                        int pal_size = (1 << st->codec->bits_per_coded_sample) << 2;
+                        const uint8_t *pal_src;
+
+                        pal_size = FFMIN(pal_size, st->codec->extradata_size);
+                        pal_src = st->codec->extradata + st->codec->extradata_size - pal_size;
 #if HAVE_BIGENDIAN
-                        for (i = 0; i < FFMIN(st->codec->extradata_size, AVPALETTE_SIZE)/4; i++)
-                            ast->pal[i] = av_bswap32(((uint32_t*)st->codec->extradata)[i]);
+                        for (i = 0; i < pal_size/4; i++)
+                            ast->pal[i] = av_bswap32(((uint32_t*)pal_src)[i]);
 #else
-                        memcpy(ast->pal, st->codec->extradata,
-                               FFMIN(st->codec->extradata_size, AVPALETTE_SIZE));
+                        memcpy(ast->pal, pal_src, pal_size);
 #endif
                         ast->has_pal = 1;
                     }