ompi_datatype_get_pack_description not thread safe

[hjelmn]

Opened a PR to fix. See #1316.

[jsquyres]

#1316 was merged to master.

@hjelmn Does this need to be PRed to v2.x?

[hjelmn]

You just merged it. open-mpi/ompi-release#909

[jsquyres]

My mistake -- this has been merged to v2.x already.

Pro tip: put "closes #1317" in the commit message to auto-close this issue when the PR is merged. 😄