Compatibility with buypass.no ACME server

At the time of this writing the buypass.no ACME server returns a valid
account Location header but an empty body in response to a request per
RFC8555 section 7.3.1 (Finding an Account URL Given a Key), even though
the RFC clearly states

    "The body of this response represents the account object as it
     existed on the server before this request"

Prior to this commit uacme followed the RFC to the letter and strictly
checked the account status from the server response body, bailing out
if not found or not valid. This made uacme incapable of requesting
certificates from buypass.no.

This commit relaxes the account status check, which is now only made if
the status field is actually there; this makes uacme tolerant of
buypass.no's RFC deviation.

uacme.c

@@ -582,10 +582,10 @@ bool account_retrieve(acme_t *a)
             acme_error(a);
             return false;
     }
-    if (json_compare_string(a->json, "status", "valid"))
+    const char* status = json_find_string(a->json, "status");
+    if (status && strcmp(status, "valid"))
     {
-        const char* status = json_find_string(a->json, "status");
-        warnx("invalid account status (%s)", status ? status : "unknown");
+        warnx("invalid account status (%s)", status);
         return false;
     }
     if (!(a->kid = find_header(a->headers, "Location")))