Fix comparison bug in shmem_sync example w/ teams

[davidozog]

This should be comparing the team handle with SHMEM_TEAM_NULL, not the PE value (my fault).

Also note that this example is also affected by #115, so we might need to precede the split with something like:

if (npes < 3) {
    printf("This example requires at least 3 PEs\n");
    shmem_global_exit(1);
}