vfs: Fix vget() error handling

The return type of vget() is int but unfortunately GCC does not complain
when we attempt to return NULL.

Spotted after converting VFS to use nullptr.

Reviewed-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
vfs: Use nullptr in C++ code

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>