Prevent redirect to Host (2) (#19175) (#19186)

Prevent redirect to Host (2) (#19175) (#19186)

Backport #19175

Unhelpfully Locations starting with `/\` will be converted by the
browser to `//` because ... well I do not fully understand. Certainly
the RFCs and MDN do not indicate that this would be expected. Providing
"compatibility" with the (mis)behaviour of a certain proprietary OS is
my suspicion. However, we clearly have to protect against this.

Therefore we should reject redirection locations that match the regular
expression: `^/[\\\\/]+`

Reference #9678

Signed-off-by: Andrew Thornton <art27@cantab.net>