net/http: fix confusing shadowing in ProxyFromEnvironment

net/http: fix confusing shadowing in ProxyFromEnvironment

The old code worked, somewhat on accident, but was confusing,
and had a useless assignment to the inner err. It worked
because url.Parse parses just about anything, so the outer err
was always nil, so it always fell through to the bottom return
statement, even without the "err = nil" line.

Instead, just have two return statements, and add a comment.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/10448044