avcodec/mjpegdec: Fix order of condition for pal8

Fixes: out of array access
Fixes: 33960/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_THP_fuzzer-5052852809629696
Fixes: 34163/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_THP_fuzzer-6123678099177472

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

libavcodec/mjpegdec.c

@@ -681,7 +681,7 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
             } else if (s->nb_components != 1) {
                 av_log(s->avctx, AV_LOG_ERROR, "Unsupported number of components %d\n", s->nb_components);
                 return AVERROR_PATCHWELCOME;
-            } else if (s->palette_index && s->bits <= 8 || s->force_pal8)
+            } else if ((s->palette_index || s->force_pal8) && s->bits <= 8)
                 s->avctx->pix_fmt = AV_PIX_FMT_PAL8;
             else if (s->bits <= 8)
                 s->avctx->pix_fmt = AV_PIX_FMT_GRAY8;