Fix (harmless) array OOB access when no pings were cached

Use `vector.data()` instead of `&vector[0]` to get access to the
underlying array.

Fixes #3850.

src/engine/client/serverbrowser_ping_cache.cpp

@@ -197,7 +197,7 @@ void CServerBrowserPingCache::GetPingCache(const CEntry **ppEntries, int *pNumEn
 		}
 		m_aNewEntries.clear();
 	}
-	*ppEntries = &m_aEntries[0];
+	*ppEntries = m_aEntries.data();
 	*pNumEntries = m_aEntries.size();
 }