Return earlier for an empty unit case. If snprintf of the fmt doesn't
produce digits between vals and ends, as happens with NaN, make the
value "none" as happens in print_metric_end.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/util/stat-display.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/tools/perf/util/stat-display.c b/tools/perf/util/stat-display.c
index 5402998881c4..e392ee5efb45 100644
--- a/tools/perf/util/stat-display.c
+++ b/tools/perf/util/stat-display.c
@@ -609,19 +609,22 @@ static void print_metric_only_json(struct perf_stat_config *config __maybe_unuse
 {
 	struct outstate *os = ctx;
 	FILE *out = os->fh;
-	char buf[64], *vals, *ends;
+	char buf[64], *ends;
 	char tbuf[1024];
+	const char *vals;
 
 	if (!valid_only_metric(unit))
 		return;
 	unit = fixunit(tbuf, os->evsel, unit);
+	if (!unit[0])
+		return;
 	snprintf(buf, sizeof(buf), fmt ?: "", val);
-	ends = vals = skip_spaces(buf);
+	vals = ends = skip_spaces(buf);
 	while (isdigit(*ends) || *ends == '.')
 		ends++;
 	*ends = 0;
-	if (!unit[0] || !vals[0])
-		return;
+	if (!vals[0])
+		vals = "none";
 	fprintf(out, "%s\"%s\" : \"%s\"", os->first ? "" : ", ", unit, vals);
 	os->first = false;
 }
-- 
2.47.0.rc1.288.g06298d1525-goog