mutex: Raise a ThreadError when detecting a fiber deadlock (#6680)

	[Bug #19105]

	If no fiber scheduler is registered and the fiber that
	owns the lock and the one that try to acquire it
	both belong to the same thread, we're in a deadlock case.

Co-authored-by: Jean Boussier <byroot@ruby-lang.org>
	---
	 test/fiber/test_mutex.rb | 22 +++++++++++++++++++++-
	 thread_sync.c            |  4 ++++
	 2 files changed, 25 insertions(+), 1 deletion(-)