perf script: Fix crash because of missing evsel->priv

perf script: Fix crash because of missing evsel->priv

'perf script' in piped mode is crashing because evsel->priv is not set
properly. Fix it.

Before:

  # perf record -o - -- ls | perf script
  <SNIP 'ls' output>
    Segmentation fault (core dumped)
  #

After:

  # perf record -o - -- ls | perf script
  <SNIP 'ls' output>
  ls 2282 1031.731974:  250000 cpu-clock:uhH:  7effe4b3d29e
  ls 2282 1031.732222:  250000 cpu-clock:uhH:  7effe4b3a650
  #

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: David Carrillo-Cisneros <davidcc@google.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Fixes: a14390fde64e ("perf script: Allow creating per-event dump files")
Link: http://lkml.kernel.org/r/20180625124220.6434-3-ravi.bangoria@linux.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>