Refactor: tools: NULL-check filename2xml() in crm_shadow

No new cts-cli tests for these cases, as I haven't figured out a way to
add them. A nonexistent file hits the check_file_exists() check. An
invalid XML file triggers a fatal assertion, but crm_shadow doesn't end
up returning CRM_EX_NOINPUT, so maybe filename2xml() isn't returning
NULL.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>