perf annotate: Add the cmp_null function and make use of it

This function exists in builtin-report.c but not in
builtin-annotate.c Functions that use cmp_null are shorter and
clearer.

Synchronizing functions between these two files will also make it
easier to potential share code in the future.

Signed-off-by: John Kacur <jkacur@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <alpine.LFD.2.00.0909241754031.11383@localhost.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>