Fix a race in the posix suspend code that would lead to a crash.

	* mono-threads-posix.c: If we get the suspend signal while the thread
	is detaching, we end up with a non null thread info but a null jit info.
	This leads to thread_state_init_from_sigctx returning false. The
	solution on this case is not to crash, but to tell the suspender
	that the current suspend attempt failed.

	Fixes an intermittent crash on the linux bots.