modbus_new_rtu Segmentation Fault when device argument is NULL

[FifteenFifty]

If the device argument to modbus_new_rtu is NULL, modbus_free is called on the context (src/modbus-rtu.c:1158).

Due to ctx->backend_data->device not having been malloc'd (this currently happens after the NULL device check), the free on line 1115 (src/modbus-rtu.c:1115) causes a segmentation fault.