Properly initialize group pointer passed to ompi_group_from_pset

The `group` pointer passed from `ompi_comm_init` is checked for
equality with MPI_GROUP_NULL in `ompi_group_from_pset` but is never
initialized. Valgrind complains about a jump based on uninitialized
values. Stack values are never default initialized so we have to
explicitly initialize it to `NULL`.

Signed-off-by: Joseph Schuchart <schuchart@icl.utk.edu>

Author: devreal

ompi/communicator/comm_init.c

@@ -192,8 +192,8 @@ int ompi_comm_init(void)
 
 int ompi_comm_init_mpi3 (void)
 {
-    ompi_group_t *group;
     int ret;
+    ompi_group_t *group = NULL;
 
     /* the intrinsic communicators have been initialized */
     ompi_comm_intrinsic_init = true;