Disable the DRC dialog while the checks are running

Closing the DRC dialog while zones are refilled leads to a crash.
Disabling the dialog prevents the crash.

pcbnew/dialogs/dialog_drc.cpp

@@ -229,6 +229,7 @@ void DIALOG_DRC_CONTROL::OnStartdrcClick( wxCommandEvent& event )
     DelDRCMarkers();
 
     wxBeginBusyCursor();
+    wxWindowDisabler disabler;
 
     // run all the tests, with no UI at this time.
     m_Messages->Clear();