Fix one leak of vImageConvertor

Author: dreampiggy

SDWebImageWebPCoder/Classes/SDImageWebPCoder.m

@@ -595,12 +595,14 @@ - (nullable NSData *)sd_encodedWebpDataWithImage:(nullable UIImage *)image quali
 
         error = vImageBuffer_Init(&dest, height, width, destFormat.bitsPerPixel, kvImageNoFlags);
         if (error != kvImageNoError) {
+            vImageConverter_Release(convertor);
             CFRelease(dataRef);
             return nil;
         }
 
         // Convert input color mode to RGB888/RGBA8888
         error = vImageConvert_AnyToAny(convertor, &src, &dest, NULL, kvImageNoFlags);
+        vImageConverter_Release(convertor);
         if (error != kvImageNoError) {
             CFRelease(dataRef);
             return nil;