Fix crates filtering box not being filled

Author: GuillaumeGomez

src/librustdoc/html/static/main.js

@@ -2422,7 +2422,7 @@ if (!DOMTokenList.prototype.remove) {
  return;
  }
  var crates_text = [];
- if (crates.length > 1) {
+ if (Object.keys(crates).length > 1) {
  for (var crate in crates) {
  if (crates.hasOwnProperty(crate)) {
  crates_text.push(crate);