[OpenMP][OMPT] Add two missing nullpointer checks in ompt-multiplex.h

In the functions ompt_multiplex_get_own_ompt_data
and ompt_multiplex_get_client_ompt_data in addition to
data being NULL, also the void pointer field "ptr" of
"data" could be NULL, leading to a subsequent
segfault.
This patch add the corresponding checks.

Patch prepared by Semih Burak

Differential Revision: https://reviews.llvm.org/D112806