Improve sanity check for DNP images

DNP images with > 256 tracks probably could be supported, but this
would require changes in multiple places where currently an uint8
is used for the track index.

The track count is limited to 255 instead of 256 to avert bugs in
loops like

  for (t=1; t<=get_param(part, LAST_TRACK); t++) {
    [...]
  }

src/d64ops.c

@@ -1303,7 +1303,7 @@ uint8_t d64_mount(path_t *path, uint8_t *name) {
     }
 
     /* Not recognized as neither D41, D71 nor D81 image; last try is DNP */
-    if ((fsize % (256*256L)) != 0) {
+    if ((fsize == 0 || fsize % (256*256L)) != 0 || fsize / (256*256L) > 255) {
       set_error(ERROR_IMAGE_INVALID);
       return 1;
     }