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