Skip PDO mapping entries with zero length.
Skip PDO mapping entries with zero length.

Some devices have a fixed number of entries (e.g. eight) in the PDO
mapping parameters and use dummy entries with zero length instead of
reducing the number-of-valid-objects entry (subindex 0).

If such a record entry is received, ignore the mapping entry in
canopen.pdo.Map.read() by not calling add_variable().

christiansandberg/canopen,christiansandberg/canopen