fix mutex memory leak

Author: kaqu

Sources/Futura/Future/Future.swift

@@ -70,14 +70,14 @@ public final class Future<Value> {
     #endif
 
     deinit {
+        defer { Mutex.destroy(mtx) }
         guard case .waiting = state else { return }
         let observers = self.observers
         executionContext.execute {
             for observer in observers {
                 observer(.canceled)
             }
         }
-        Mutex.destroy(mtx)
     }
 }