c->out can hold a valid picture on bitstream errors after 4903d87b
when frame multi-threading is used. Output this picture instead of
returning the return value of dav1d_parse_obus().

Fixes `assert(dst->data[0] == ((void*)0));` in dav1d_picture_ref with
clusterfuzz-testcase-minimized-dav1d_fuzzer_mt-5743306491822080. Also
fixes a memory leak of frames with
clusterfuzz-testcase-minimized-dav1d_fuzzer_mt-5655593017147392. Credits
to oss-fuzz.