gh-87389: Fix an open redirection vulnerability in http.server. (GH-93879) (GH-94093)

Fix an open redirection vulnerability in the `http.server` module when
an URI path starts with `//` that could produce a 301 Location header
with a misleading target.  Vulnerability discovered, and logic fix
proposed, by Hamza Avvan (@hamzaavvan).

Test and comments authored by Gregory P. Smith [Google].
(cherry picked from commit 4abab6b6)

Co-authored-by: Gregory P. Smith <greg@krypto.org>