perf tools: Ensure comm string is properly terminated

If threads in a multi-threaded process have names shorter than the main
thread the comm for the named threads is not properly terminated.

E.g., for the process 'namedthreads' where each thread is named noploop%d
where %d is the thread number:

Before:
    perf script -f comm,tid,ip,sym,dso
    noploop:4ads 21616  400a49 noploop (/tmp/namedthreads)
The 'ads' in the thread comm bleeds over from the process name.

After:
    perf script -f comm,tid,ip,sym,dso
       noploop:4 21616  400a49 noploop (/tmp/namedthreads)

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1330111898-68071-1-git-send-email-dsahern@gmail.com


Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>