videoio: improve condition for MJPEG gstreamer sink

modules/videoio/src/cap_gstreamer.cpp

@@ -178,6 +178,8 @@ protected:
     bool          isPosFramesSupported;
     bool          isPosFramesEmulated;
     gint64        emulatedFrameNumber;
+
+    bool          isOutputByteBuffer;
 };
 
 /*!
@@ -205,6 +207,8 @@ void CvCapture_GStreamer::init()
     isPosFramesSupported = false;
     isPosFramesEmulated = false;
     emulatedFrameNumber = -1;
+
+    isOutputByteBuffer = false;
 }
 
 /*!
@@ -357,6 +361,7 @@ IplImage * CvCapture_GStreamer::retrieveFrame(int)
         } else if(strcasecmp(name, "image/jpeg") == 0) {
             depth = 1;
             // the correct size will be set once the first frame arrives
+            isOutputByteBuffer = true;
         }
 #endif
         if (depth > 0) {
@@ -383,7 +388,8 @@ IplImage * CvCapture_GStreamer::retrieveFrame(int)
     gboolean success = gst_buffer_map(buffer,&info, (GstMapFlags)GST_MAP_READ);
 
     // with MJPEG streams frame size can change arbitrarily
-    if(int(info.size) != frame->imageSize) {
+    if (isOutputByteBuffer && (size_t)info.size != (size_t)frame->imageSize)
+    {
         cvReleaseImageHeader(&frame);
         frame = cvCreateImageHeader(cvSize(info.size, 1), IPL_DEPTH_8U, 1);
     }