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>