fixed memory leak with PassthroughRelay

Author: just1103

Sources/Relays/PassthroughRelay.swift

@@ -74,14 +74,15 @@ private extension PassthroughRelay {
         func forceFinish() {
             self.sink?.shouldForwardCompletion = true
             self.sink?.receive(completion: .finished)
+            self.sink = nil
         }
 
         func request(_ demand: Subscribers.Demand) {
             sink?.demand(demand)
         }
 
         func cancel() {
-            sink = nil
+            forceFinish()
         }
     }
 }