efi_loader: memory leak efi_add_memory_map_pg

Don't leak newlist if we error out.

Fixes: 74c16acc ("efi_loader: Don't allocate from memory holes")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>