Make sure fs.dirname() doesn't truncate the last character of the
returned path. Previously ucv_string_new_length was called with a
length which no longer included the last character (which had just
been tested not to be a '/' or '.' and hence broke the loop at that
point).

-- Testcase --
{%
	fs = require('fs');
	printf("%s\n", fs.dirname('/etc/config/wireless'));
%}
-- End --

-- Expect stdout --
/etc/config
-- End --