perf ui hists: Guard against NULL hist_entry in add_script_opt()

In add_script_opt(), when the browser selection has no valid hist_entry
(for example, when all entries are filtered out or the trace is empty),
hist_browser__selected_entry() returns NULL.

Add an explicit check for he == NULL prior to accessing he->time to avoid a
potential NULL pointer dereference crash.

Assisted-by: Antigravity:gemini-3.5-flash
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/perf/ui/browsers/hists.c

@@ -2826,6 +2826,8 @@ add_script_opt(struct hist_browser *browser,
 			time_optstr++;
 			time_act++;
 		}
+		if (he == NULL)
+			return n;
 		j = sprintf(tstr, " in ");
 		j += timestamp__scnprintf_usec(he->time, tstr + j,
 					       sizeof tstr - j);