Merge pull request #819 from n13/master

guarding against image source == NULL

SDWebImage/UIImage+MultiFormat.m

@@ -47,6 +47,7 @@
 +(UIImageOrientation)sd_imageOrientationFromImageData:(NSData *)imageData {
     UIImageOrientation result = UIImageOrientationUp;
     CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)imageData, NULL);
+    if (imageSource) {
         CFDictionaryRef properties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);
         if (properties) {
             CFTypeRef val;
@@ -61,6 +62,7 @@
             //NSLog(@"NO PROPERTIES, FAIL");
         }
         CFRelease(imageSource);
+    }
     return result;
 }