vfs/cbe_trust_anchor: add sanity check

This patch adds a null-pointer check to the '_close_handle' method,
which triggers when using the vfs_cbe.run script in interactive mode.

repos/gems/src/lib/vfs/cbe_trust_anchor/vfs.cc

@@ -544,6 +544,9 @@ class Trust_anchor
 
 		void _close_handle(Vfs::Vfs_handle **handle)
 		{
+			if (*handle == nullptr)
+				return;
+
 			(*handle)->close();
 			(*handle) = nullptr;
 		}