Make parallel video writer test pass if compiled without threading support

modules/highgui/test/test_ffmpeg.cpp

@@ -241,7 +241,9 @@ public:
 
     virtual void operator() (const Range& range) const
     {
-        CV_Assert((range.start + 1) == range.end);
+        if((range.start + 1) != range.end)
+            return;
+
         VideoWriter* writer = writers->operator[](range.start);
         CV_Assert(writer != NULL);
         CV_Assert(writer->isOpened());
@@ -303,7 +305,9 @@ public:
 
     virtual void operator() (const Range& range) const
     {
-        CV_Assert(range.start + 1 == range.end);
+        if((range.start + 1) != range.end)
+            return;
+
         VideoCapture* capture = readers->operator[](range.start);
         CV_Assert(capture != NULL);
         CV_Assert(capture->isOpened());