perf callchain: Add missing parent_val initialization

Adding missing parent_val callchain_node initialization.
It's causing segfault in perf top:

  $ sudo perf top -g
  perf: Segmentation fault
  -------- backtrace --------
  free_callchain_node(+0x29) in perf [0x4a4b3e]
  free_callchain(+0x29) in perf [0x4a5a83]
  hist_entry__delete(+0x126) in perf [0x4c6649]
  hists__delete_entry(+0x6e) in perf [0x4c66dc]
  hists__decay_entries(+0x7d) in perf [0x4c6776]
  perf_top__sort_new_samples(+0x7c) in perf [0x436a78]
  hist_browser__run(+0xf2) in perf [0x507760]
  perf_evsel__hists_browse(+0x1da) in perf [0x507c8d]
  perf_evlist__tui_browse_hists(+0x3e) in perf [0x5088cf]
  display_thread_tui(+0x7f) in perf [0x437953]
  start_thread(+0xc5) in libpthread-2.21.so [0x7f7068fbb555]
  __clone(+0x6d) in libc-2.21.so [0x7f7066fc3b9d]
  [0x0]

Reported-and-Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 4b3a3212 ("perf hists browser: Support flat callchains")
Link: http://lkml.kernel.org/r/20151121102355.GA17313@krava.local


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