Prevent 500 error when search wiki for non-existing repo

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

CHANGELOG

@@ -24,6 +24,7 @@ v 7.3.0
   - Comment & Close button
   - Process git push --all much faster
   - Don't allow edit of system notes
+  - Project wiki search (Ralf Seidler)
 v 7.2.1
   - Delete orphaned labels during label migration (James Brooks)

lib/gitlab/project_search_results.rb

@@ -49,11 +49,16 @@ module Gitlab
     end
     def wiki_blobs
-      if !project.wiki_enabled?
-        []
+      if project.wiki_enabled?
+        wiki_repo = Repository.new(ProjectWiki.new(project).path_with_namespace)
+
+        if wiki_repo.exists?
+          wiki_repo.search_files(query)
+        else
+          []
+        end
       else
-        Repository.new(ProjectWiki.new(project).path_with_namespace).
-           search_files(query)
+        []
       end
     end