perf stat: Avoid sending SIGTERM to random processes

This patch fixes a problem with perf stat whereby on termination it may
send a SIGTERM signal to random processes on systems with high PID
recycling. I got some actual bug reports on this.

There is race between the SIGCHLD and sig_atexit() handlers.  This patch
addresses this problem by clearing child_pid in the SIGCHLD handler.

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20130604154426.GA2928@quad
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>