Fix corruption of thread linked list (crystal-lang#8196)

When a new thread finishes very quickly it could happen that
the removal from the linked list was executed before the push.
The linked list didn't check for that situation and it ends corrupted.

src/thread.cr

@@ -34,9 +34,7 @@ class Thread
       Pointer(Void).null
     }, self.as(Void*))
 
-    if ret == 0
-      Thread.threads.push(self)
-    else
+    if ret != 0
       raise Errno.new("pthread_create", ret)
     end
   end
@@ -124,6 +122,7 @@ class Thread
   end
 
   protected def start
+    Thread.threads.push(self)
     Thread.current = self
     @main_fiber = fiber = Fiber.new(stack_address, self)