Pcbnew: release the lock file if a board was loaded, and when clearing the board currently edited.

When the board is cleared, the previous loaded board is no longer in edit.

pcbnew/files.cpp

@@ -460,7 +460,7 @@ bool PCB_EDIT_FRAME::OpenProjectFiles( const std::vector<wxString>& aFileSet, in
     }
 
     // Release the lock file, until the new file is actually loaded
-    m_file_checker.reset( nullptr );
+    ReleaseFile();
 
     wxFileName pro = fullFileName;
     pro.SetExt( ProjectFileExtension );

pcbnew/initpcb.cpp

@@ -48,6 +48,9 @@ bool PCB_EDIT_FRAME::Clear_Pcb( bool aQuery )
             return false;
     }
 
+    // Release the lock file, if exists
+    ReleaseFile();
+
     // Clear undo and redo lists because we want a full deletion
     GetScreen()->ClearUndoRedoList();
     GetScreen()->ClrModify();