Améliorations

• J1939 database updated to 2017-04
• Some minor internal improvements and fixes

Corrections de bugs

• Error Frame description for "Error passive flag" was missing
• If no data length was specified for a J1939PG multipacket symbol, any variable located beyond data byte 7 was not initialized with its default value
• String variables with a length greater than 8 bytes in multipacket J1939 symbols were only decoded when the variable had an SPN value assigned