the mp_log is freed when the corresponding mpv_handle (ta_parent) is
freed in the EventHelper, though it could still be used from different
threads. instead don't use a ta_parent and manually free on manual
dereferencing. on app shutdown (memory cleanup) this is not called but
instead is freed by the usual cleanup and freeing.
the LogHelper is only able to be manually dereferenced in the AppHub,
so no race conditions are possible in all other cases (vo).in the AppHub
it's impossible to hit a race condition atm, because of how the init
process works and how/where the log is used. only manually forcing
logging in the exit process itself could theoretically trigger a use
after free.
Fixes#13823
der richter
e42a8d537f