Fix a bug in multi VFD when the memb_addr in H5Pset_fapl_multi is passed
as NULL. The library is supposed in that case to equally divide the
address space among all members, but there was a bug causing an
overflow in the assignment.

tested with h5commitest



git-svn-id: https://svn.hdfgroup.uiuc.edu/hdf5/trunk@25620 dab4d1a6-ed17-0410-a064-d1ae371a2980