only remove old sqlite database if it exists

lib/setup.php

@@ -131,7 +131,9 @@ class OC_SETUP {
 			}
 			else {
 				//delete the old sqlite database first, might cause infinte loops otherwise
-				unlink("$datadir/owncloud.db");
+				if(file_exists("$datadir/owncloud.db")){
+					unlink("$datadir/owncloud.db");
+				}
 				//in case of sqlite, we can always fill the database
 				OC_DB::createDbFromStructure('db_structure.xml');
 			}