avcodec/h264_slice: Check sps in h264_slice_header_init()

Fixes: null pointer dereference
Fixes: h264_slice_header_init.mp4

Found-by: Rafael Dutra <rafael.dutra@cispa.de>
Tested-by: Rafael Dutra <rafael.dutra@cispa.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_slice.c

@@ -922,6 +922,11 @@ static int h264_slice_header_init(H264Context *h)
     const SPS *sps = h->ps.sps;
     int i, ret;
 
+    if (!sps) {
+        ret = AVERROR_INVALIDDATA;
+        goto fail;
+    }
+
     ff_set_sar(h->avctx, sps->sar);
     av_pix_fmt_get_chroma_sub_sample(h->avctx->pix_fmt,
                                      &h->chroma_x_shift, &h->chroma_y_shift);