Declare VirtViewerFile variable at start of method
    
    virt_viewer_session_vnc_auth_credential uses gotos which jump over the
    declaration of 'file', meaning its contents are uninitialized in the
    jump target.
    
    Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>