random vlc_join error with vlc_executor
Reproduced by executing ./test_libvlc_slaves in loop.
FAIL: test_libvlc_slaves
========================
LibVLC fatal error joining thread (35) in thread 51972 at ../../src/posix/thread.c:227 in vlc_join
/usr/lib/libasan.so.6(+0x45a3e)[0x7effab06ea3e]
/home/alexandre/workspace/videolabs/vlc/build-native/src/.libs/libvlccore.so.9(+0x24c47a)[0x7effaaeb047a]
/home/alexandre/workspace/videolabs/vlc/build-native/src/.libs/libvlccore.so.9(+0x24cbd9)[0x7effaaeb0bd9]
/home/alexandre/workspace/videolabs/vlc/build-native/src/.libs/libvlccore.so.9(vlc_join+0xf9)[0x7effaaeb0e79]
/home/alexandre/workspace/videolabs/vlc/build-native/src/.libs/libvlccore.so.9(vlc_executor_Delete+0x2c7)[0x7effaae72cc7]
/home/alexandre/workspace/videolabs/vlc/build-native/src/.libs/libvlccore.so.9(+0xed7e3)[0x7effaad517e3]
/home/alexandre/workspace/videolabs/vlc/build-native/src/.libs/libvlccore.so.9(libvlc_InternalCleanup+0x1df)[0x7effaacfd80f]
/home/alexandre/workspace/videolabs/vlc/build-native/lib/.libs/libvlc.so.12(libvlc_release+0x6b)[0x7effaafefacb]
/home/alexandre/workspace/videolabs/vlc/build-native/lib/.libs/libvlc.so.12(libvlc_media_release+0x2aa)[0x7effaaffc3ba]
/home/alexandre/workspace/videolabs/vlc/build-native/lib/.libs/libvlc.so.12(+0x2186f)[0x7effaaffc86f]
/home/alexandre/workspace/videolabs/vlc/build-native/src/.libs/libvlccore.so.9(+0xebe7b)[0x7effaad4fe7b]
/home/alexandre/workspace/videolabs/vlc/build-native/src/.libs/libvlccore.so.9(+0x20e2fd)[0x7effaae722fd]
/usr/lib/libpthread.so.0(+0x93e9)[0x7effaaa733e9]
/usr/lib/libc.so.6(clone+0x43)[0x7effaa9a1293]
Thread error: Success
FAIL test_libvlc_slaves (exit status: 134)
