Fix the wrong changes of previous commit

Author: dreampiggy

SDWebImagePDFCoder/Classes/SDImagePDFCoder.m

@@ -193,9 +193,8 @@ + (NSData *)createPDFDataWithBitmapImage:(UIImage *)image {
     CGContextDrawImage(context, mediaBox, imageRef);
     CGContextEndPage(context);
 
-    UIImage *result = [pdfData copy];
+    NSData *result = [pdfData copy];
 
-    CGImageRelease(imageRef);
     CGDataConsumerRelease(pdfConsumer);
     CGContextRelease(context);