even more robust data_type check

v1/libtpcimgio/ecat63r.c

@@ -67,7 +67,7 @@ int ecat63ReadMainheader(
   if(!little) swabip(buf+50, 2);
   memcpy(&h->data_type, buf+50, 2); 
   if(ECAT63_TEST>10) printf("main_header.data_type=%d\n", h->data_type);
-  if(h->data_type==-1) {
+  if(h->data_type<1) {
     if(ECAT63_TEST>1) printf("invalid data_type; assuming VAX_I2\n");
     h->data_type=VAX_I2;
   }