perf hists: hist_entry__cmp() may use he_tmp.hists, initialize it

perf hists: hist_entry__cmp() may use he_tmp.hists, initialize it

The iter_add_next_cumulative_entry() function calls hist_entry__cmp(),
which may want to access the hists where this hist_entry is stored,
initialize it to let that happen and avoid segfaults.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-iqg98sfn4fvwcxp0pdvqauie@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>