Protect against missing guard in check_return.

Author: samuel-williams-shopify

lib/async/safe/monitor.rb

@@ -159,6 +159,9 @@ def transfer(*objects)
 				@mutex.synchronize do
 					entry = @guards[object]
 
+					# No guard held, nothing to release:
+					return unless entry
+					
 					if safe == false
 						# Simple tracking (single guard)
 						# Release if this fiber holds it