Fix incorrect area calculation for polygon (fix #14675)

(cherry-picked from bf4cf51)

Author: nyalldawson

src/core/qgsdistancearea.cpp

@@ -931,7 +931,8 @@ double QgsDistanceArea::computePolygonArea( const QList<QgsPoint>& points ) cons
     dx = x2 - x1;
     area += dx * ( m_Qp - getQ( y2 ) );
 
-    if (( dy = y2 - y1 ) != 0.0 )
+    dy = y2 - y1;
+    if ( !qgsDoubleNear( dy, 0.0 ) )
       area += dx * getQ( y2 ) - ( dx / dy ) * ( Qbar2 - Qbar1 );
   }
   if (( area *= m_AE ) < 0.0 )

tests/src/core/testqgsdistancearea.cpp

@@ -43,6 +43,7 @@ class TestQgsDistanceArea: public QObject
     void measureUnits();
     void measureAreaAndUnits();
     void emptyPolygon();
+    void regression14675();
 
 };
 
@@ -356,6 +357,17 @@ void TestQgsDistanceArea::emptyPolygon()
   da.measurePolygon( QList< QgsPoint >() );
 }
 
+void TestQgsDistanceArea::regression14675()
+{
+  //test regression #14675
+  QgsDistanceArea calc;
+  calc.setEllipsoidalMode( true );
+  calc.setEllipsoid( "GRS80" );
+  calc.setSourceCrs( 145L );
+  QgsGeometry geom( QgsGeometryFactory::geomFromWkt( "Polygon ((917593.5791854317067191 6833700.00807378999888897, 917596.43389983859378844 6833700.67099479306489229, 917599.53056440979707986 6833700.78673478215932846, 917593.5791854317067191 6833700.00807378999888897))" ) );
+  QVERIFY( qgsDoubleNear( calc.measureArea( &geom ), 0.83301, 0.0001 ) );
+}
+
 QTEST_MAIN( TestQgsDistanceArea )
 #include "testqgsdistancearea.moc"