Merge pull request #22 from SDWebImage/fix_animated_webp_frame_blend_issue

Fix the Animated WebP coder the frame blend index calculation issue, he end condition should be only `endIndex` to match the behavior.

Author: kinarobin

SDWebImageWebPCoder/Classes/SDImageWebPCoder.m

@@ -841,7 +841,7 @@ - (UIImage *)safeAnimatedImageFrameAtIndex:(NSUInteger)index {
                 @autoreleasepool {
                     [self sd_blendWebpImageWithCanvas:_canvas iterator:iter colorSpace:_colorSpace];
                 }
-            } while ((size_t)iter.frame_num < (endIndex + 1) && WebPDemuxNextFrame(&iter));
+            } while ((size_t)iter.frame_num < endIndex && WebPDemuxNextFrame(&iter));
         }
     }