ffv1: check for malloc failure

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

libavcodec/ffv1.c

@@ -123,6 +123,10 @@ av_cold int ffv1_init_slice_contexts(FFV1Context *f)
         int sxe         = f->avctx->width  * (sx + 1) / f->num_h_slices;
         int sys         = f->avctx->height *  sy      / f->num_v_slices;
         int sye         = f->avctx->height * (sy + 1) / f->num_v_slices;
+
+        if (!fs)
+            return AVERROR(ENOMEM);
+
         f->slice_context[i] = fs;
         memcpy(fs, f, sizeof(*fs));
         memset(fs->rc_stat2, 0, sizeof(fs->rc_stat2));