slugs: Fix regex for legacy stream slugs.

This prevents a bug where we interpret "2something"
as a modern slug instead of a legacy stream name.

The bug was probably somewhat unlikely to happen in
practice, since it only manifests if 2 is an actual
stream_id.

static/js/stream_data.js

@@ -328,7 +328,7 @@ exports.slug_to_name = function (slug) {
     GitHub conversations.  We migrated to modern slugs in
     early 2018.
     */
-    const m = /^(\d+)(-.*)?/.exec(slug);
+    const m = /^(\d+)(-.*)?$/.exec(slug);
     if (m) {
         const stream_id = Number.parseInt(m[1], 10);
         const sub = subs_by_stream_id.get(stream_id);