Avoid crashing if a new thread is signaled right away

Avoid crashing if a new thread is signaled right away

* src/thread.c (post_acquire_global_lock): Don't raise the pending
signal if the thread's handlers were not yet set up, as that will
cause Emacs to exit with a fatal error.  This can happen if a
thread is signaled as soon as make-thread returns, before the new
thread had an opportunity to acquire the global lock, set up the
handlers, and call the thread function.

* test/src/thread-tests.el (thread-signal-early): New test.