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Correct the condition for freeing references in dav1d_parse_obus

A new coded video sequence (see page 193; section 7.5 of the spec)
begins when we see a sequence header that isn't bit identical to
previous ones. This is the point at which we can throw away previous
frames etc.

--- a/src/obu.c

+++ b/src/obu.c

@@ -1090,7 +1090,10 @@

             return res;

         if (check_for_overrun(&gb, init_bit_pos, len))

             return -EINVAL;

-        if (!c->have_frame_hdr || memcmp(&hdr, &c->seq_hdr, sizeof(hdr))) {

+        // If we have read a sequence header which is different from

+        // the old one, this is a new video sequence and can't use any

+        // previous state. Free that state.

+        if (c->have_seq_hdr && memcmp(&hdr, &c->seq_hdr, sizeof(hdr))) {

             for (int i = 0; i < 8; i++) {

                 if (c->refs[i].p.p.data[0])

                     dav1d_thread_picture_unref(&c->refs[i].p);

@@ -1099,8 +1102,7 @@

                 if (c->cdf[i].cdf)

                     dav1d_cdf_thread_unref(&c->cdf[i]);

             }

-            if (c->have_seq_hdr)

-                c->seq_hdr = hdr;

+            c->seq_hdr = hdr;

         }

         c->have_seq_hdr = 1;

         break;