Ensure lock is activated when editing subscriptions

Author: jabeattie

Sources/Subjects/ReplaySubject.swift

@@ -72,6 +72,9 @@ public final class ReplaySubject<Output, Failure: Error>: Subject {
         }
 
         subscriptions.forEach { $0.forwardCompletionToBuffer(completion) }
+
+        lock.lock()
+        defer { self.lock.unlock() }
         self.subscriptions.removeAll()
     }