Fix crash when calculating offset on certain single curves which beco…

…me multilinestrings when offset

src/core/geometry/qgsgeometry.cpp

@@ -1805,12 +1805,15 @@ QgsGeometry QgsGeometry::offsetCurve( double distance, int segments, JoinStyle j
       return result;
     }
 
-    const QgsCurve::Orientation newOrientation = qgsgeometry_cast< const QgsCurve * >( offsetGeom.get() )->orientation();
-    if ( newOrientation != prevOrientation )
+    if ( const QgsCurve *offsetCurve = qgsgeometry_cast< const QgsCurve * >( offsetGeom.get() ) )
     {
-      // GEOS has flipped line orientation, flip it back
-      std::unique_ptr< QgsAbstractGeometry > flipped( qgsgeometry_cast< const QgsCurve * >( offsetGeom.get() )->reversed() );
-      offsetGeom = std::move( flipped );
+      const QgsCurve::Orientation newOrientation = offsetCurve->orientation();
+      if ( newOrientation != prevOrientation )
+      {
+        // GEOS has flipped line orientation, flip it back
+        std::unique_ptr< QgsAbstractGeometry > flipped( offsetCurve->reversed() );
+        offsetGeom = std::move( flipped );
+      }
     }
     return QgsGeometry( std::move( offsetGeom ) );
   }

tests/src/python/test_qgsgeometry.py

@@ -4601,6 +4601,8 @@ def testOffsetCurve(self):
             ["LINESTRING (0 0, 0 100, 100 100)", -1, "LineString (1 0, 1 99, 100 99)"],
             ["LINESTRING (100 100, 0 100, 0 0)", 1, "LineString (100 99, 1 99, 1 0)"],
             ["LINESTRING (100 100, 0 100, 0 0)", -1, "LineString (100 101, -1 101, -1 0)"],
+            # linestring which becomes multilinestring -- the actual offset curve calculated by GEOS looks bad, but we shouldn't crash here
+            ["LINESTRING (259329.820 5928370.79, 259324.337 5928371.758, 259319.678 5928372.33, 259317.064 5928372.498 )", 100, "MultiLineString ((259313.3 5928272.5, 259312.5 5928272.6),(259312.4 5928272.3, 259309.5 5928272.8, 259307.5 5928273.1))"],
             ["MULTILINESTRING ((0 0, 0 100, 100 100),(100 100, 0 100, 0 0))", 1, "MultiLineString ((-1 0, -1 101, 100 101),(100 99, 1 99, 1 0))"]
         ]
         for t in tests: