[FIX] DB: Don't fail when we don't need to back propagate

Edge case: all the queue has the same priority, do the get_previous_prio_diff failed and it was treated as an error.

src/database/queue.c

@@ -579,14 +579,14 @@ ___database_back_propagate_priority(lkt_db *db)
     sqlite3_stmt *stmt                 = NULL;
 
     /* Currently playing position */
-    SQLITE_PREPARE(db, stmt, SQL_GET_CURRENT, not_playing_or_no_need_to_back_propagate);
+    SQLITE_PREPARE(db, stmt, SQL_GET_CURRENT, error);
     SQLITE_STEP_ROW(db, stmt, not_playing_or_no_need_to_back_propagate);
     currently_playing_position = sqlite3_column_int64(stmt, 0);
     sqlite3_finalize(stmt);
 
     /* Next kara with priority > 1 */
     SQLITE_PREPARE(db, stmt, SQL_GET_POS_WITH_PRIO_SUP_1, error);
-    SQLITE_STEP_ROW(db, stmt, error);
+    SQLITE_STEP_ROW(db, stmt, not_playing_or_no_need_to_back_propagate);
     next_kara_with_prio_sup_1  = sqlite3_column_int64(stmt, 0);
     priority_to_back_propagate = sqlite3_column_int(stmt, 1);
     sqlite3_finalize(stmt);