check for validity, not just against (0, 0) coordinates
check for validity, not just against (0, 0) coordinates

* (0, 0) are in fact valid coordinates and shouldn't be used to indicate "invalid" coordinates

tzapzoor/marble,quannt24/marble,probonopd/marble,quannt24/marble,David-Gil/marble-dev,David-Gil/marble-dev,probonopd/marble,tucnak/marble,quannt24/marble,tzapzoor/marble,David-Gil/marble-dev,tucnak/marble,probonopd/marble,tzapzoor/marble,David-Gil/marble-dev,David-Gil/marble-dev,tucnak/marble,tucnak/marble,tzapzoor/marble,quannt24/marble,tzapzoor/marble,tzapzoor/marble,tucnak/marble,tucnak/marble,quannt24/marble,David-Gil/marble-dev,quannt24/marble,tzapzoor/marble,probonopd/marble,tzapzoor/marble,probonopd/marble,probonopd/marble,tucnak/marble,quannt24/marble,probonopd/marble