perf tools: Introduce xzalloc() for detecting out of memory conditions

perf tools: Introduce xzalloc() for detecting out of memory conditions

Introducing xzalloc() which wrapping zalloc() for detecting out
of memory conditions.

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: systemtap <systemtap@sources.redhat.com>
Cc: DLE <dle-develop@lists.sourceforge.net>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100316220521.32050.85155.stgit@localhost6.localdomain6>
[ -v2: small cleanups in surrounding code ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>