Clear selection before deleting DRC markers

Pcbnew crashes when a selected DRC marker is deleted. This a possible
fix to lp:#1773863.

pcbnew/dialogs/dialog_drc.cpp

@@ -39,6 +39,9 @@
 #include <class_draw_panel_gal.h>
 #include <view/view.h>
 
+#include <tool/tool_manager.h>
+#include <tools/pcb_actions.h>
+
 /* class DIALOG_DRC_CONTROL: a dialog to set DRC parameters (clearance, min cooper size)
  * and run DRC tests
  */
@@ -649,6 +652,10 @@ void DIALOG_DRC_CONTROL::RedrawDrawPanel()
 void DIALOG_DRC_CONTROL::DelDRCMarkers()
 {
     m_brdEditor->SetCurItem( NULL );           // clear curr item, because it could be a DRC marker
+
+    // Clear current selection list to avoid selection of deleted items
+    m_brdEditor->GetToolManager()->RunAction( PCB_ACTIONS::selectionClear, true );
+
     m_ClearanceListBox->DeleteAllItems();
     m_UnconnectedListBox->DeleteAllItems();
     m_DeleteCurrentMarkerButton->Enable( false );