On startup, remove feed entries with empty URL on startup

This should never happen, but occasionally did when something went wrong
while adding a new feed, e.g. a crash.  This would then cause a segfault
on startup.

BUG: 495961

src/database.cpp

@@ -348,5 +348,10 @@ void Database::setWalMode()
 
 void Database::cleanup()
 {
-    // TODO: create database sanity checks, or, alternatively, create database scrub routine
+    // delete rows with empty feed urls, as this should never happen, but could
+    // occur when something goes wrong (like a crash) when trying to add a new
+    // feed
+    QSqlQuery query;
+    query.prepare(QStringLiteral("DELETE FROM Feeds WHERE url is NULL or url='';"));
+    execute(query);
 }