blkparse: skip check_cpu_map with pipe input

blkparse: skip check_cpu_map with pipe input

When we're using pipe input, we don't track online CPUs and don't have a
cpu_map.  When we start to show entries, check_sequence will be invoked.
If the first entry isn't sequence 1 (perhaps it's been dropped?), we'll
proceed to check_cpu_map.  Since we haven't tracked online CPUs,
pdi->cpu_map_max will be 0 and we'll do a malloc(0).  Then we'll start
setting bits corresponding to CPU numbers in memory we don't own.  Since
there's nothing to check here, let's skip it on pipe input.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>