perf tools: Assign default value for some pointers

Assign default value for pointers that are identified by the compiler as
non-initialized.

Signed-off-by: Martin Liska <mliska@suse.cz>
Acked-by: Ingo Molnar <mingo@kernel.org>
Link: http://lkml.kernel.org/r/5564393C.1090104@suse.cz
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>