iffdec: Fix integer overflow.

Found-by: durandal_1707
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/iff.c

@@ -238,6 +238,11 @@ static int extract_header(AVCodecContext *const avctx,
                 s->mask_buf = av_malloc((s->planesize * 32) + FF_INPUT_BUFFER_PADDING_SIZE);
                 if (!s->mask_buf)
                     return AVERROR(ENOMEM);
+                if (s->bpp > 16) {
+                    av_log(avctx, AV_LOG_ERROR, "bpp %d too large for palette\n", s->bpp);
+                    av_freep(&s->mask_buf);
+                    return AVERROR(ENOMEM);
+                }
                 s->mask_palbuf = av_malloc((2 << s->bpp) * sizeof(uint32_t) + FF_INPUT_BUFFER_PADDING_SIZE);
                 if (!s->mask_palbuf) {
                     av_freep(&s->mask_buf);