benchmarks/imb: Fix P_Write_Shared segfault
With CHECK_RESULTS enabled, IMB-IO-openmpi P_Write_Shared crashes on
the first non-zero message size because IMB_chk_distr() uses reccnt,
while IMB_init_pointers() leaves it NULL for MPIIO builds.
Allocate the arrays needed for MPIIO builds.
PR: 296182
Reviewed by: thierry (mentor)
Approved by: thierry (mentor)