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>
0b1686b67a