MDL-56127 forms: multiple select dependencies fixed

This is a fix for a regressions introduced by MDL-54778. It
seems that escaped all our tests because there isn't any case
of such a dependency in core, but Daniele Cordella was facing
it within his surveypro module.

lib/form/form.js

@@ -496,6 +496,7 @@ if (typeof M.form.dependencyManager === 'undefined') {
                     // Multiple selects can have one or more value assigned. A pipe (|) is used as a value separator
                     // when multiple values have to be selected at the same time.
                     values = value.split('|');
+                    selected = [];
                     this.get('options').each(function() {
                         if (this.get('selected')) {
                             selected[selected.length] = this.get('value');