avcodec/h264_slice: ignore SAR changes in slices after the first

Fixes race condition and null pointer dereference
Fixes: signal_sigsegv_1472ac3_468_cov_2915641226_CABACI3_Sony_B.jsv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 38d5241b7f)

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

libavcodec/h264_slice.c

@@ -1431,13 +1431,15 @@ int ff_h264_decode_slice_header(H264Context *h, H264Context *h0)
                      || 16*h->sps.mb_height * (2 - h->sps.frame_mbs_only_flag) != h->avctx->coded_height
                      || h->avctx->bits_per_raw_sample != h->sps.bit_depth_luma
                      || h->cur_chroma_format_idc != h->sps.chroma_format_idc
-                     || av_cmp_q(h->sps.sar, h->avctx->sample_aspect_ratio)
                      || h->mb_width  != h->sps.mb_width
                      || h->mb_height != h->sps.mb_height * (2 - h->sps.frame_mbs_only_flag)
                     ));
     if (non_j_pixfmt(h0->avctx->pix_fmt) != non_j_pixfmt(get_pixel_format(h0, 0)))
         must_reinit = 1;
 
+    if (first_slice && av_cmp_q(h->sps.sar, h->avctx->sample_aspect_ratio))
+        must_reinit = 1;
+
     h->mb_width  = h->sps.mb_width;
     h->mb_height = h->sps.mb_height * (2 - h->sps.frame_mbs_only_flag);
     h->mb_num    = h->mb_width * h->mb_height;