Bug 1682639 fixed an assertion failure in PrepareDatastoreOp::NestedRun in case the quota manager has no usage information, but the database file already existed nonetheless. The fix was to assume a 0 size in this case. However, it should be checked again if this is right behaviour. At least, the debug-mode assertion should be removed if this is really a possible case at this place. A debug-mode assertion wouldn't make sense here, as the inconsistency results from external influences (failure to remove the database file).