lib: explicity free the metadata buffers in dav1d_close()

They were meant to be freed by the call to dav1d_flush() at the beginning,
but before the previous commit that wasn't the case when decoding with one
frame thread.
This ensures they are explicity freed here and not depend on other functions
behaving correctly.

src/lib.c

@@ -535,6 +535,9 @@ static void close_internal(Dav1dContext **const c_out, int flush) {
     dav1d_ref_dec(&c->seq_hdr_ref);
     dav1d_ref_dec(&c->frame_hdr_ref);
 
+    dav1d_ref_dec(&c->mastering_display_ref);
+    dav1d_ref_dec(&c->content_light_ref);
+
     dav1d_freep_aligned(c_out);
 }