perf tools: Fix buffer allocation

"perf top" cores dump on my dev machine, if run from a directory
where vmlinux is present:

  *** glibc detected *** malloc(): memory corruption: 0x085670d0 ***

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: <stable@kernel.org>
LKML-Reference: <4ABB6EB7.7000002@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>