frame mt: fix memleak caused by race between dav1d_close and dav1d_decode_frame
frame mt: fix memleak caused by race between dav1d_close and dav1d_decode_frame

The race is exposed by not draining the decoder correctly after
026069693ef (decoupled decoding api). Fixes a memleak with
clusterfuzz-testcase-minimized-dav1d_fuzzer_mt-5728508249112576. Credits
to oss-fuzz.

videolan/dav1d