This commit fixes #4662 - "Portals4/MTL hangs in c_get_accumulate test".

- Portals4/OSC was unable to acquire an exclusive lock due to an invalid
local address in the atomic operation.  This caused the reported hang.
- After fixing the hang, the test continued to fail because
ompi_datatype_is_contiguous_memory_layout() reports that MPI_EMPTY (the
origin datatype) is noncontiguous and Portals4/OSC does not support
noncontiguous datatypes at this time.  However, in this case the origin
count is zero so contiguous/noncontiguous is irrelevant.  Now we skip
the contiguous check if the count is zero.

cmr=v1.8.3:reviewer=regrant:subject=Fix for "Portals4/MTL hangs in c_get_accumulate test"
Add Todd Kordenbrock to the AUTHORS file.