create_pthread() crashes if pthread_create() failed

[dkogan]

Look here:

fprime/Os/Posix/Task.cpp

Line 161 in 91c578e

stat = pthread_create(tid, &att, pthread_entry_wrapper, arg);

If pthread_create() fails, fprime checks for failure. But if it did fail, it tries to pthread_join(). THIS causes a segfault. There is no valid thread (because we couldn't create it), so any operation on it will crash.