Replacing #781 - based on http://blog.benjamin-encz.de/post/main-queue-vs-main-t…

…hread/, background queue can execute code on the main thread, so we need to check for the main queue to assure safety

@@ -107,7 +107,7 @@ extern NSString *const SDWebImageErrorDomain;
     }
 
 #define dispatch_main_async_safe(block)\
-    if ([NSThread isMainThread]) {\
+    if (strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(dispatch_get_main_queue())) == 0) {\
         block();\
     } else {\
         dispatch_async(dispatch_get_main_queue(), block);\