avcodec/libdav1d: fix null pointer dereference in LCEVC side data handling

ff_frame_new_side_data() may set sd to NULL and return 0 when
side_data_pref() determines that existing side data should be
preferred.

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavcodec/libdav1d.c

@@ -459,6 +459,8 @@ FF_ENABLE_DEPRECATION_WARNINGS
                                          bytestream2_get_bytes_left(&gb), &sd);
             if (res < 0)
                 return res;
+            if (!sd)
+                break;
 
             bytestream2_get_bufferu(&gb, sd->data, sd->size);
             break;