avcodec/iff: Only write palette to plane 1 if its PAL8

Fixes: null pointer passed as argument 1, which is declared to never be null
Fixes: 33791/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_IFF_ILBM_fuzzer-5107575256383488.fuzz

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/iff.c

@@ -1847,7 +1847,8 @@ static int decode_frame(AVCodecContext *avctx,
                     buf += s->planesize;
                 }
             }
-            memcpy(frame->data[1], s->pal, 256 * 4);
+            if (avctx->pix_fmt == AV_PIX_FMT_PAL8)
+                memcpy(frame->data[1], s->pal, 256 * 4);
         } else if (s->ham) {
             int i, count = 1 << s->ham;