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Fix parsing segmentation data in parse_frame_hdr

The first memset is dead code: if primary_ref_frame is
PRIMARY_REF_NONE then segmentation.update_data is always true. The
patch removes this memset and explains why the copy in the other
branch is correct.

The second memset should always fire: if segmentation is not enabled
for this frame, the seg_data structure should be set to zero rather
than copied from a reference frame (see section 5.9.14 of the AV1
spec).

--- a/src/obu.c

+++ b/src/obu.c

@@ -616,21 +616,17 @@

                     hdr->segmentation.seg_data.preskip = 1;

                 }

             }

-        } else if (hdr->primary_ref_frame == PRIMARY_REF_NONE) {

-            memset(&hdr->segmentation.seg_data, 0, sizeof(Av1SegmentationDataSet));

-            for (int i = 0; i < NUM_SEGMENTS; i++)

-                hdr->segmentation.seg_data.d[i].ref = -1;

         } else {

+            // segmentation.update_data was false so we should copy

+            // segmentation data from the reference frame.

+            assert(hdr->primary_ref_frame != PRIMARY_REF_NONE);

             const int pri_ref = hdr->refidx[hdr->primary_ref_frame];

             hdr->segmentation.seg_data = c->refs[pri_ref].seg_data;

         }

-    } else if (hdr->primary_ref_frame == PRIMARY_REF_NONE) {

+    } else {

         memset(&hdr->segmentation.seg_data, 0, sizeof(Av1SegmentationDataSet));

         for (int i = 0; i < NUM_SEGMENTS; i++)

             hdr->segmentation.seg_data.d[i].ref = -1;

-    } else {

-        const int pri_ref = hdr->refidx[hdr->primary_ref_frame];

-        hdr->segmentation.seg_data = c->refs[pri_ref].seg_data;

     }

 #if DEBUG_FRAME_HDR

     printf("HDR: post-segmentation: off=%ld\n",