Fix type assertions from directory_url expression result

cmd/csaf_checker/processor.go

@@ -918,7 +918,7 @@ func (p *processor) checkCSAFs(domain string) error {
 		p.badProviderMetadata.warn("extracting directory URLs failed: %v.", err)
 	} else {
 		var ok bool
-		dirURLs, ok = directoryURLs.([]string)
+		dirURLs, ok = util.AsStrings(directoryURLs)
 		if !ok {
 			p.badProviderMetadata.warn("directory URLs are not strings.")
 		}

csaf/advisories.go

@@ -153,7 +153,7 @@ func (afp *AdvisoryFileProcessor) Process(
 			lg("extracting directory URLs failed: %v\n", err)
 		} else {
 			var ok bool
-			dirURLs, ok = directoryURLs.([]string)
+			dirURLs, ok = util.AsStrings(directoryURLs)
 			if !ok {
 				lg("directory_urls are not strings.\n")
 			}

util/json.go

@@ -166,3 +166,19 @@ func (pe *PathEval) Strings(
 	}
 	return results, nil
 }
+
+// AsStrings transforms an []interface{string, string,... }
+// to a []string.
+func AsStrings(x interface{}) ([]string, bool) {
+	strs, ok := x.([]interface{})
+	if !ok {
+		return nil, false
+	}
+	res := make([]string, 0, len(strs))
+	for _, y := range strs {
+		if s, ok := y.(string); ok {
+			res = append(res, s)
+		}
+	}
+	return res, true
+}