perf report: Fix NULL pointer dereference in hists__fprintf_nr_sample_events()

The 'evname' variable can be NULL, as it is checked a few lines back,
check it before using.

Fixes: 9e207ddf ("perf report: Show call graph from reference events")
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/Signed-off-by: Gaurav Singh <gaurav1086@gmail.com>

tools/perf/builtin-report.c

@@ -478,8 +478,7 @@ static size_t hists__fprintf_nr_sample_events(struct hists *hists, struct report
 	if (rep->time_str)
 		ret += fprintf(fp, " (time slices: %s)", rep->time_str);
 
-	if (symbol_conf.show_ref_callgraph &&
-	    strstr(evname, "call-graph=no")) {
+	if (symbol_conf.show_ref_callgraph && evname && strstr(evname, "call-graph=no")) {
 		ret += fprintf(fp, ", show reference callgraph");
 	}