player/scripting: fix race condition when destroying clients

Clients run in detached threads and are waited to signal when they are
finished with self mpv_destroy() call. Problem is that after detached
thread calls this function, mpv consider this client to be done and can
do whatever. One example is when mpv exits and wait for scripts to
finish we got race condition, because mpv (main thread) can exit first
and the detached thread calls ta_free() which crashes, because we may
have already destroyed things that the thread depends on.

I've seen this crash on FreeBSD CI job (libmpv-lifetime test), where the
libmpv.so has been unloaded while the thread was still doing ta_free()

```
==5081==ERROR: AddressSanitizer: SEGV on unknoun address 0x000001111806 (pc 0x000001111806 bp 0x7fffdedf4e40 sp 0x7fffdedf4e08 T40)
==5081==The signal is caused by a READ memory access.
    #0 0x1111806  (<unknown module>)
    #1 0x803cd6900 in ta_free /home/runner/work/mpv/mpv/build/../ta/ta.c:243:5
    #2 0x803a2fc87 in run_script /home/runner/work/mpv/mpv/build/../player/scripting.c:93:5
    #3 0x803a2ffe0 in script_thread /home/runner/work/mpv/mpv/build/../player/scripting.c:99:5
    #4 0x2c451a in asan_thread_start(void*) /usr/src/contrib/llvm-project/compiler-rt/lib/asan/asan_interceptors.cpp:239:28
    #5 0x80034fb04  (/lib/libthr.so.3+0x10b04)
```

Note the 0x1111806 jump which is completelly bogus.

Fix this by doing mpv_destroy() as a last step in detached threads.

Author: kasper93

input/ipc-win.c

@@ -303,8 +303,9 @@ static MP_THREAD_VOID client_thread(void *p)
         CloseHandle(arg->write_ol.hEvent);
 
     CloseHandle(arg->client_h);
-    mpv_destroy(arg->client);
+    mpv_handle *client = arg->client;
     talloc_free(arg);
+    mpv_destroy(client);
     MP_THREAD_RETURN();
 }
 

player/scripting.c

@@ -89,8 +89,9 @@ static void run_script(struct mp_script_args *arg)
     if (arg->backend->load(arg) < 0)
         MP_ERR(arg, "Could not load %s script %s\n", arg->backend->name, arg->filename);
 
-    mpv_destroy(arg->client);
+    mpv_handle *client = arg->client;
     talloc_free(arg);
+    mpv_destroy(client);
 }
 
 static MP_THREAD_VOID script_thread(void *p)