If we use an unsupported transport (e.g., http when curl
support is not compiled in), transport_get reports an error
to the user, but we still get a transport object. We need to
manually check and abort the clone process at that point, or
we end up with a segfault.
Noticed by Thomas Rast.
Signed-off-by: Jeff King <peff@peff.net> Acked-by: Daniel Barkalow <barkalow@iabervon.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>