perf report TUI: Fix segmentation fault in perf_evsel__hists_browse()

perf report TUI: Fix segmentation fault in perf_evsel__hists_browse()

The segmentation fault can be reproduced as following steps:

1) Executing perf report in tui.

2) Typing '/xxxxx' to filter the symbol to get nothing matched.

3) Pressing enter with no entry selected.

Then it will report a segmentation fault.

It is caused by the lack of check of browser->he_selection when
accessing it's member res_samples in perf_evsel__hists_browse().

These processes are meaningful for specified samples, so we can skip
these when nothing is selected.

Fixes: 3d7a161b64d9 ("perf report: Implement browsing of individual samples")
Signed-off-by: Wei Li <liwei391@huawei.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
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: Hanjun Guo <guohanjun@huawei.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: http://lore.kernel.org/lkml/20200612094322.39565-1-liwei391@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>