install: fix NameError

If `len(args)` is less than two, then
`install_path = dst_dir + node_prefix + '/'` would throw a `NameError`,
because `dst_dir` will not be defined yet. So we are assigning `''` as
the default value.

PR-URL: https://github.com/iojs/io.js/pull/1628


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>