[Libguestfs] [PATCH libnbd v3] lib/errors.c: Fix assert fail in exit path in multi-threaded code

This version simply removes the call to pthread_key_destroy. It fixes the problem and allows us to leave the asserts alone so we can still catch real errors. Of course this leaks pthread_key_t in the case where you dlclose() the library. glibc has a limit of 128 thread-specific keys (and the first 32 are somehow "fast" in a way I could quite follow), so that's a thing. Rich.