Improved error handling if trying to release an unrelated resource.

lib/async/pool/controller.rb

@@ -264,7 +264,7 @@ def reuse(resource)
 				Console.debug(self) {"Reuse #{resource}"}
 				usage = @resources[resource]
 
-				if usage.zero?
+				if usage.nil? || usage.zero?
 					raise "Trying to reuse unacquired resource: #{resource}!"
 				end
 

test/async/pool/controller.rb

@@ -22,6 +22,14 @@
 			expect(pool).to be(:empty?)
 		end
 
+		it 'raises an error when releasing an unacquired resource' do
+			resource = Async::Pool::Resource.new
+
+			expect do
+				pool.release(resource)
+			end.to raise_exception(RuntimeError, message: be =~ /unacquired resource/)
+		end
+
 		with '#as_json' do
 			it 'generates a JSON representation' do
 				expect(pool.as_json).to be == {