adding test for `polylines()` call with empty Point-s vector and fix for crash in this case

modules/core/src/drawing.cpp

@@ -2215,7 +2215,10 @@ void cv::polylines(InputOutputArray _img, InputArrayOfArrays pts,
     {
         Mat p = pts.getMat(manyContours ? i : -1);
         if( p.total() == 0 )
+        {
+            npts[i] = 0;
             continue;
+        }
         CV_Assert(p.checkVector(2, CV_32S) >= 0);
         ptsptr[i] = (Point*)p.data;
         npts[i] = p.rows*p.cols*p.channels()/2;

modules/core/test/test_misc.cpp

@@ -48,3 +48,23 @@ TEST(Core_SaturateCast, NegativeNotClipped)
 
     ASSERT_EQ(0xffffffff, val);
 }
+
+TEST(Core_Drawing, polylines_empty)
+{
+    Mat img(100, 100, CV_8UC1, Scalar(0));
+    vector<Point> pts; // empty
+    polylines(img, pts, false, Scalar(255));
+    int cnt = countNonZero(img);
+    ASSERT_EQ(cnt, 0);
+}
+
+TEST(Core_Drawing, polylines)
+{
+    Mat img(100, 100, CV_8UC1, Scalar(0));
+    vector<Point> pts;
+    pts.push_back(Point(0, 0));
+    pts.push_back(Point(20, 0));
+    polylines(img, pts, false, Scalar(255));
+    int cnt = countNonZero(img);
+    ASSERT_EQ(cnt, 21);
+}