player/lua: fix autofree ctx leak on exit() call

Fixes LSAN leak when tests call fail() and exit(1) is called. We have
mpv_destroy set for atexit but data.ctx was null parented, so it had no
parent to be freed by. Parent it to script_ctx instead so it gets
cleaned up by mpv_destroy.

LSAN output:

Direct leak of 400 byte(s) in 5 object(s) allocated from:
    #\0 0x7f4c6c52c01f in malloc (/lib64/libasan.so.8+0x12c01f) (BuildId: db0aa5bf57ed53a5869a3a2cc49051292ceb6390)
    #\1 0x7f4c6b276388 in ta_alloc_size ../ta/ta.c:139
    #\2 0x7f4c6b2bf01f in script_autofree_trampoline ../player/lua.c:1310
    #\3 0x7f4c69373b15  (/lib64/libluajit-5.1.so.2+0xbb15) (BuildId: 182622c63dea3dd74173bbd5a810051a76bcf4d9)