Fixs several problems found by static analysis

modules/calib3d/src/chessboard.cpp

@@ -2215,11 +2215,13 @@ int Chessboard::Board::detectMarkers(cv::InputArray image)
     cv::Mat mask = cv::Mat::zeros(DUMMY_FIELD_SIZE,DUMMY_FIELD_SIZE,CV_8UC1);
     cv::circle(mask,cv::Point(DUMMY_FIELD_SIZE/2,DUMMY_FIELD_SIZE/2),DUMMY_FIELD_SIZE/7,cv::Scalar::all(255),-1);
     int signal_size = cv::countNonZero(mask);
+    CV_Assert(signal_size > 0);
 
     cv::Mat mask2 = cv::Mat::zeros(DUMMY_FIELD_SIZE,DUMMY_FIELD_SIZE,CV_8UC1);
     cv::circle(mask2,cv::Point(DUMMY_FIELD_SIZE/2,DUMMY_FIELD_SIZE/2),DUMMY_FIELD_SIZE/2,cv::Scalar::all(255),-1);
     cv::circle(mask2,cv::Point(DUMMY_FIELD_SIZE/2,DUMMY_FIELD_SIZE/2),DUMMY_FIELD_SIZE/5,cv::Scalar::all(0),-1);
     int noise_size = cv::countNonZero(mask2);
+    CV_Assert(noise_size > 0);
 
     std::vector<cv::Point2f> dst,src;
     dst.push_back(cv::Point2f(0.0F,0.0F));
@@ -3369,7 +3371,7 @@ cv::Scalar Chessboard::Board::calcEdgeSharpness(cv::InputArray _img,float rise_d
                 value += patch.at<uint8_t>(0,0);
                 ++count2;
             }
-            values.push_back(uint8_t(value/count2));
+            values.push_back(count2 > 0 ? uint8_t(value/count2) : 0);
         }
 
         float val = calcSharpness(values,rise_distance);

modules/core/src/persistence.cpp

@@ -704,6 +704,7 @@ public:
             const char* json_signature = "{";
             const char* xml_signature  = "<?xml";
             char* buf = this->gets(16);
+            CV_Assert(buf);
             char* bufPtr = cv_skip_BOM(buf);
             size_t bufOffset = bufPtr - buf;
 

modules/features2d/src/kaze/AKAZEFeatures.cpp

@@ -1323,6 +1323,7 @@ void quantized_counting_sort(const float a[], const int n,
                              const float quantum, const int nkeys,
                              int idx[/*n*/], int cum[/*nkeys + 1*/])
 {
+  CV_Assert(nkeys > 0);
   memset(cum, 0, sizeof(cum[0]) * (nkeys + 1));
 
   // Count up the quantized values

modules/objdetect/src/qrcode.cpp

@@ -1593,6 +1593,8 @@ bool QRDetectMulti::checkPoints(const vector<Point2f>& quadrangle_points)
             li2++;
         }
     }
+    if (count_w == 0)
+        return false;
 
     double frac = double(count_b) / double(count_w);
     double bottom_bound = 0.76;