[frontend] Fix rendering invalid markdown links

Since commit c466c65 we overwrite
the redcarpet renderer for links and add the OBS domain to relative
links.
The code that would craft these URIs wasn't expecting invalid links
and would fail with an URI::InvalidURIError exception.
This commit catches such exceptions and renders the invalid markdown
in the same format Redcarpet would do.

src/api/lib/obsapi/markdown_renderer.rb

@@ -27,7 +27,10 @@ def preprocess(fulldoc)
     # unfortunately we can't call super (into C) - see vmg/redcarpet#51
     def link(link, title, content)
       title = " title='#{title}'" if title.present?
-      link = URI.join(::Configuration.obs_url, link)
+      begin
+        link = URI.join(::Configuration.obs_url, link)
+      rescue URI::InvalidURIError
+      end
       "<a href='#{link}'#{title}>#{content}</a>"
     end
   end

src/api/spec/helpers/comment_helper_spec.rb

@@ -0,0 +1,23 @@
+require 'rails_helper'
+
+RSpec.describe CommentHelper, type: :helper do
+  describe '#comment_body' do
+    it 'renders markdown links to html links' do
+      expect(comment_body('[my link](https://github.com/openSUSE/open-build-service/issues/5091)')).to eq(
+        "<p><a href='https://github.com/openSUSE/open-build-service/issues/5091'>my link</a></p>\n"
+      )
+    end
+
+    it 'adds the OBS domain to relative links' do
+      expect(comment_body('[my link](/here)')).to eq(
+        "<p><a href='#{::Configuration.obs_url}/here'>my link</a></p>\n"
+      )
+    end
+
+    it 'does not crash due to invalid URIs' do
+      expect(comment_body("anbox[400000+22d000]\r\n(the number)")).to eq(
+        "<p>anbox<a href='the number'>400000+22d000</a></p>\n"
+      )
+    end
+  end
+end