transport: fix leak with transport helper URLs

transport: fix leak with transport helper URLs

Transport URLs can be prefixed with "foo::", which would tell us that
the transport uses a remote helper called "foo". We extract the helper
name by `xstrndup()`ing the prefix before the double-colons, but never
free that string.

Fix this leak by assigning the result to a separate local variable that
we can then free upon returning.

Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>