When writing MP4 sample, save buffer.size() into a temporary variable before calling buffer.extract(). (#2628)

src/zm_video.cpp

@@ -101,7 +101,7 @@ X264MP4Writer::X264MP4Writer(
     Error("Failed init swscaleobj");
     return;
   }
-    
+
   swscaleobj.SetDefaults(zm_pf, codec_pf, width, height);
 
   /* Calculate the image sizes. We will need this for parameter checking */
@@ -458,11 +458,12 @@ int X264MP4Writer::x264encodeloop(bool bFlush) {
 
       /* Write the sample */
       if ( !buffer.empty() ) {
+        unsigned int bufSize = buffer.size();
         if ( !MP4WriteSample(
               mp4h,
               mp4vtid,
-              buffer.extract(buffer.size()),
-              buffer.size(),
+              buffer.extract(bufSize),
+              bufSize,
               duration,
               offset,
               prevKeyframe) ) {