lib/index: fix deadlock when mu4e-index-cleanup is nil (#1749)

If the user has wants to postpone clean-up we shouldn't lock the
indexer waiting for something that will never happen. Clear the flag
event though we are actually skipping cleanup.

lib/index/mu-indexer.cc

@@ -270,6 +270,9 @@ Indexer::Private::start(const Indexer::Config& conf)
                         cleanup();
                         clean_done_ = true;
                         g_debug ("cleanup finished");
+                } else {
+                        clean_done_ = true;
+                        g_debug ("cleanup skipped");
                 }
 
                 store_.commit();