h4h5tools

Fix many warnings GCC gives!

* remove all unused variable

* initialize uninitialized variables
Sometimes, this is not trivial because of complex flow. For memory allocation
and release, I made all uses and releases check the validity of the pointer so
that double-free or accessing invalid address will not happen.  For HDF5
function pair like H5Gopen() and H5Gclose(), I believe that HDF5 checks whether
or not the argument of H5Gclose() is valid; so, I just initialized the
descriptor with -1.

* Some of them were false positives, but I tried to make gcc happy as much as possible.