perf hists: Fix crash in perf_hpp__reset_output_field()

perf hists: Fix crash in perf_hpp__reset_output_field()

Du Changbin reported crash [1] when calling perf_hpp__reset_output_field()
after unregistering field via perf_hpp__column_unregister().

This ends up in calling following list_del* sequence on
the same format:

  perf_hpp__column_unregister:
    list_del(&format->list);
  perf_hpp__reset_output_field:
    list_del_init(&fmt->list);

where the later list_del_init might touch already freed formats.

Fixing this by replacing list_del() with list_del_init() in
perf_hpp__column_unregister().

[1] http://marc.info/?l=linux-kernel&m=149059595826019&w=2

Reported-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20171013083736.15037-2-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>