make watershed demo work correctly

modules/core/src/drawing.cpp

@@ -2033,7 +2033,7 @@ cvDrawContours( void* _img, CvSeq* contour,
     scalarToRawData( externalColor, ext_buf, img.type(), 0 );
     scalarToRawData( holeColor, hole_buf, img.type(), 0 );
 
-    maxLevel = MAX(maxLevel, INT_MIN-2);
+    maxLevel = MAX(maxLevel, INT_MIN+2);
     maxLevel = MIN(maxLevel, INT_MAX-1);
     
     if( maxLevel < 0 )

samples/cpp/watershed.cpp

@@ -86,6 +86,8 @@ int main( int argc, char** argv )
             
             findContours(markerMask, contours, hierarchy, CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE);
             
+            if( contours.empty() )
+                continue;
             Mat markers(markerMask.size(), CV_32S);
             markers = Scalar::all(0);
             int idx = 0;