Check if faces detected or not.

Added if condition, whether faces are detected or not, will help to keep program running for some images.
If imwrite used along with imshow, it may produce error for :  No faces found.Aborting.. in function 'fit' (Bad Argument).

modules/face/samples/sampleDetectLandmarks.cpp

@@ -75,19 +75,26 @@ int main(int argc,char** argv){
     resize(img,img,Size(460,460), 0, 0, INTER_LINEAR_EXACT);
     facemark->getFaces(img,faces);
     vector< vector<Point2f> > shapes;
-    if(facemark->fit(img,faces,shapes))
-    {
-        for( size_t i = 0; i < faces.size(); i++ )
+
+    // Check if faces detected or not
+    // Helps in proper exception handling when writing images to the directories.
+    if(faces.size() != 0) {
+        if(facemark->fit(img,faces,shapes))
         {
-            cv::rectangle(img,faces[i],Scalar( 255, 0, 0 ));
-        }
-        for(unsigned long i=0;i<faces.size();i++){
-            for(unsigned long k=0;k<shapes[i].size();k++)
-                cv::circle(img,shapes[i][k],5,cv::Scalar(0,0,255),FILLED);
+            for( size_t i = 0; i < faces.size(); i++ )
+            {
+                cv::rectangle(img,faces[i],Scalar( 255, 0, 0 ));
+            }
+            for(unsigned long i=0;i<faces.size();i++){
+                for(unsigned long k=0;k<shapes[i].size();k++)
+                    cv::circle(img,shapes[i][k],5,cv::Scalar(0,0,255),FILLED);
+            }
+            namedWindow("Detected_shape");
+            imshow("Detected_shape",img);
+            waitKey(0);
         }
-        namedWindow("Detected_shape");
-        imshow("Detected_shape",img);
-        waitKey(0);
+    } else {
+        cout << "Faces not detected." << endl;
     }
     return 0;
-}
+}