Fall-back to internal selection value if selection is a string

When DomainParser format a domain from a DomainValidationError, the field
definition may contain as selection as string. At this place we can not really
make an other RPC call to convert the selection so we fall-back to the internal
value.

issue9989
review349271002

src/common.js

@@ -2005,9 +2005,11 @@
                 return (value * factor).toFixed(digit);
             };
             var format_selection = function() {
-                for (var i = 0; i < field.selection.length; i++) {
-                    if (field.selection[i][0] == value) {
-                        return field.selection[i][1];
+                if (field.selection instanceof Array) {
+                    for (var i = 0; i < field.selection.length; i++) {
+                        if (field.selection[i][0] == value) {
+                            return field.selection[i][1];
+                        }
                     }
                 }
                 return value || '';