check_signature_token broken in merge commit ed87738e

The check_signature_token function (in src/or/routerparse.c) was broken in merge commit ed87738ede789fb: the string "< digest_len" was replaced with "< DIGEST_LEN" on line 1090 (see the last hunk of the diff from the first parent of that commit for src/or/routerparse.c).

This is currently not a problem, as the function's parameter digest_len is always equal to DIGEST_LEN, but we should fix it anyway.