Fix crash if merge geometries fails

Author: mhugent

src/app/qgisapp.cpp

@@ -4765,17 +4765,16 @@ QgsGeometry* QgisApp::unionGeometries( const QgsVectorLayer* vl, QgsFeatureList&
     {
       backupPtr = unionGeom;
       unionGeom = unionGeom->combine( currentGeom );
-      if ( !unionGeom )
-      {
-        delete backupPtr;
-        QApplication::restoreOverrideCursor();
-        return 0;
-      }
       if ( i > 1 ) //delete previous intermediate results
       {
         delete backupPtr;
         backupPtr = 0;
       }
+      if ( !unionGeom )
+      {
+        QApplication::restoreOverrideCursor();
+        return 0;
+      }
     }
   }
 

src/core/qgsgeometry.cpp

@@ -6565,6 +6565,11 @@ QgsGeometry* QgsGeometry::combine( QgsGeometry* geometry )
   try
   {
     GEOSGeometry* unionGeom = GEOSUnion( mGeos, geometry->mGeos );
+    if ( !unionGeom )
+    {
+      return 0;
+    }
+
     if ( type() == QGis::Line )
     {
       GEOSGeometry* mergedGeom = GEOSLineMerge( unionGeom );