When asked to both use a .netrc file for credentials and to follow HTTP
redirects, curl could leak the password used for the first host to the
followed-to host under certain circumstances.

This flaw only manifests itself if the netrc file matches the redirect target
hostname, but the entry omits password or both login & password.

A curl transfer with `a.com` that redirects to `b.com` that uses a `.netrc`
like below (with a match, but no password specified for the second host),
would make curl pass on `alicespassword` as password even in the second
transfer to the separate host `b.com`.

~~~
machine a.com
   login alice
   password alicespassword

default
   login bob
~~~