hdf5_ep

[svn-r8899] 
Purpose:
Bug Fix

Description:
Calling H5Sset_extent_simple to change a dataspace's maxdims from nonzero to
zero causes errors (infinite loops, seg faults, asserts) because the pointer
to the maximum size isn't cleaned up properly

Solution:
Clean up that pointer.  Added a test for this case.

Platforms tested:
sleipnir (very minor change)