h4h5tools

Fixed a bug occurring when using a dimension map with negative increment and non-one offset

The problem was revealed while writing and testing the HDF-EOS2 handler for OpenDAP by valgrind memcheck.

This problem only happens only if following conditions are met:
  - a shrunken geo-location needs to be created due to a dimension map with a negative increment value
  - the offset value of that dimension map is equal to or greater than 2

When those conditions are met, the following happens:
  - a data array representing a geo-location field is accessed with an illegal index
    * if a user is very lucky, this invalid access results in segmentation fault.
    - else
       * if a user is very unlucky, the read value can be NaN, and this results in a real failure
       - else, the read value is multipled by 0, which means no matter what value is read, the result is just 0

Only (*)-marked case can result in wrong conversion.