curl: Fix error path in curl_open()

curl: Fix error path in curl_open()

g_hash_table_destroy() and g_hash_table_foreach_remove() (called by
curl_drop_all_sockets()) both require the table to be non-NULL, or will
print assertion failures (just print, no abort).

There are several paths in curl_open() that can lead to the out_noclean
label without s->sockets being allocated, so clean it only if it has
been allocated.

Example reproducer:
$ qemu-img info -f http ''
qemu-img: GLib: g_hash_table_foreach_remove: assertion 'hash_table != NULL' failed
qemu-img: GLib: g_hash_table_destroy: assertion 'hash_table != NULL' failed
qemu-img: Could not open '': http curl driver cannot handle the URL '' (does not start with 'http://')

Closes: https://gitlab.com/qemu-project/qemu/-/issues/1475
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20230206132949.92917-1-hreitz@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>