Check on fscanf in read_mask of vpx_temporal_svc_encoder

Follow-up to:
https://chromium-review.googlesource.com/c/webm/libvpx/+/6489527

Bug: b:412867390
Change-Id: If5043d2b73024d7f00178880e0d4c43787905c00
1 file changed