Fix measure dialog shows wrong results when changing units

If the measurement was not finished (ie right click occurred)
then the last length would be missing from the table

Fix #15433

(cherry-picked from 03bff50)

src/app/qgsmeasuredialog.cpp

@@ -111,6 +111,13 @@ void QgsMeasureDialog::unitsChanged( int index )
   mTable->clear();
   mTotal = 0.;
   updateUi();
+
+  if ( !mTool->done() )
+  {
+    // re-add temporary mouse cursor position
+    addPoint( mLastMousePoint );
+    mouseMove( mLastMousePoint );
+  }
 }
 
 void QgsMeasureDialog::restart()