perf diff: Fix segfault on 'perf diff -o N' option

perf diff: Fix segfault on 'perf diff -o N' option

The -o/--order option is to select column number to sort a diff result.

It does the job by adding a hpp field at the beginning of the sort list.
But it should not be added to the output field list as it has no
callbacks required by a output field.

During the setup_sorting(), the perf_hpp__setup_output_field() appends
the given sort keys to the output field if it's not there already.

Originally it was checked by fmt->list being non-empty.  But commit
39721c45f513 ("perf hists: Make hpp setup function generic") changed it
to check the ->equal callback.

Anyways, we don't need to add the pseudo hpp field to the output field
list since it won't be used for output.  So just skip fields if they
have no ->color or ->entry callbacks.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Fixes: 39721c45f513 ("perf hists: Make hpp setup function generic")
Link: http://lkml.kernel.org/r/20170118051457.30946-1-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>