remote-curl.c: free memory in cmd_main()

remote-curl.c: free memory in cmd_main()

Plug a trivial memory leak in code added in a2d725b7bdf (Use an
external program to implement fetching with curl, 2009-08-05).

To do this have the cmd_main() use a "goto cleanup" pattern, and to
return an error of 1 unless we can fall through to the http_cleanup()
at the end.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>