perf evlist: Fix use of uninitialized variable

perf evlist: Fix use of uninitialized variable

Fixes valgrind complaint:
==1870== Syscall param write(buf) points to uninitialised byte(s)
==1870==    at 0x4E3F5B0: __write_nocancel (in /lib64/libpthread-2.14.90.so)
==1870==    by 0x449D7C: perf_evlist__start_workload (evlist.c:846)
==1870==    by 0x427BC1: cmd_record (builtin-record.c:561)
==1870==    by 0x419D72: run_builtin (perf.c:319)
==1870==    by 0x4195F2: main (perf.c:376)
==1870==  Address 0x7feffcdd7 is on thread 1's stack

Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1372793245-4136-3-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>