[Cabana] Fix segfault in sparkline (#30870)

tools/cabana/chart/sparkline.cc

@@ -13,7 +13,11 @@ void Sparkline::update(const MessageId &msg_id, const cabana::Signal *sig, doubl
   auto first = std::lower_bound(msgs.cbegin(), msgs.cend(), first_ts, CompareCanEvent());
   auto last = std::upper_bound(first, msgs.cend(), ts, CompareCanEvent());
 
-  if (first != last && !size.isEmpty()) {
+  if (first == last || size.isEmpty()) {
+    pixmap = QPixmap();
+    return;
+  }
+
   points.clear();
   double value = 0;
   for (auto it = first; it != last; ++it) {
@@ -21,15 +25,18 @@ void Sparkline::update(const MessageId &msg_id, const cabana::Signal *sig, doubl
       points.emplace_back(((*it)->mono_time - (*first)->mono_time) / 1e9, value);
     }
   }
+
+  if (points.empty()) {
+    pixmap = QPixmap();
+    return;
+  }
+
   const auto [min, max] = std::minmax_element(points.begin(), points.end(),
                                               [](auto &l, auto &r) { return l.y() < r.y(); });
   min_val = min->y() == max->y() ? min->y() - 1 : min->y();
   max_val = min->y() == max->y() ? max->y() + 1 : max->y();
   freq_ = points.size() / std::max(points.back().x() - points.front().x(), 1.0);
   render(sig->color, range, size);
-  } else {
-    pixmap = QPixmap();
-  }
 }
 
 void Sparkline::render(const QColor &color, int range, QSize size) {