Fix crash under iOS 6 when dispatch_release is called (see #213)

SDWebImage/SDImageCache.m

@@ -91,7 +91,7 @@ static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
 - (void)dealloc
 {
     [[NSNotificationCenter defaultCenter] removeObserver:self];
-    dispatch_release(_ioQueue);
+    SDDispatchQueueRelease(_ioQueue);
 }
 
 #pragma mark SDImageCache (private)

SDWebImage/SDWebImageCompat.h

@@ -33,6 +33,19 @@
 #import <UIKit/UIKit.h>
 #endif
 
+// @see https://github.com/ccgus/fmdb/commit/aef763eeb64e6fa654e7d121f1df4c16a98d9f4f
+#define SDDispatchQueueRelease(q) (dispatch_release(q))
+
+#if TARGET_OS_IPHONE
+    #if __IPHONE_OS_VERSION_MIN_REQUIRED >= 60000
+        #define SDDispatchQueueRelease(q)
+    #endif
+#else
+    #if MAC_OS_X_VERSION_MIN_REQUIRED >= 1080
+        #define SDDispatchQueueRelease(q)
+    #endif
+#endif
+
 NS_INLINE UIImage *SDScaledImageForPath(NSString *path, NSObject *imageOrData)
 {
     if (!imageOrData)

SDWebImage/SDWebImageDownloader.m

@@ -77,8 +77,8 @@ NSString *const kCompletedCallbackKey = @"completed";
 - (void)dealloc
 {
     [self.downloadQueue cancelAllOperations];
-    dispatch_release(_workingQueue);
-    dispatch_release(_barrierQueue);
+    SDDispatchQueueRelease(_workingQueue);
+    SDDispatchQueueRelease(_barrierQueue);
 }
 
 - (void)setMaxConcurrentDownloads:(NSInteger)maxConcurrentDownloads