Clear dimension pointer after committing.

Fixes https://gitlab.com/kicad/code/kicad/issues/11864

(cherry picked from commit b80fd5329d)

pcbnew/tools/drawing_tool.cpp

@@ -989,6 +989,7 @@ int DRAWING_TOOL::DrawDimension( const TOOL_EVENT& aEvent )
 
             if( ++step >= FINISHED )
             {
+                dimension = nullptr;
                 step = SET_ORIGIN;
                 m_controls->SetAutoPan( false );
                 m_controls->CaptureCursor( false );