macOS: Fixed a data race in URL connections

modules/juce_core/native/juce_mac_Network.mm

@@ -331,7 +331,8 @@ public:
     NSMutableData* data = nil;
     NSDictionary* headers = nil;
     int statusCode = 0;
-    bool initialised = false, hasFailed = false, hasFinished = false, isBeingDeleted = false;
+    std::atomic<bool> initialised { false };
+    bool hasFailed = false, hasFinished = false, isBeingDeleted = false;
     const int numRedirectsToFollow;
     int numRedirects = 0;
     int64 latestTotalBytes = 0;