[mc] Fix smpi_process_index_of_smx_process()

[mc] Fix smpi_process_index_of_smx_process()

The SMPI process ID is simply the SIMIX PID - 1. The previous solution
(getting the data field of the SIMIX process) was mostly correct but
the 'data' field was not initally set to the SMPI process data
structure (but was NULL).