Fix writing empty fields of structures which are themselves structures
Fix writing empty fields of structures which are themselves structures

* src/mat5.c (WriteCompressedStructField): Check
      matvar->internal->num_fields to determine if the structure is empty

martinichka/matio-coder,MaartenBent/matio,CBL-ORION/matio,CBL-ORION/matio-sf-mirror,NJannasch/matio,tbeu/matio,tbeu/matio,NJannasch/matio,tbeu/matio,telehan/libmatio,CBL-ORION/matio,MaartenBent/matio,telehan/libmatio,MaartenBent/matio,MaartenBent/matio,CBL-ORION/matio-sf-mirror,tbeu/matio,martinichka/matio-coder,martinichka/matio-coder