fix edge case when cache is empty, fix codestyle

fixes segfault in Cache::getSurroundingInterval

include/message_filters/cache.hpp

@@ -209,6 +209,12 @@ class Cache : public SimpleFilter<M>
     namespace mt = message_filters::message_traits;
 
     std::lock_guard<std::mutex> lock(cache_lock_);
+    if (cache_.size() == 0) 
+    {
+      // early return for empty cache; Logic below is only valid for caches with at least one element.
+      return {};
+    }
+
     // Find the starting index. (Find the first index after [or at] the start of the interval)
     int start_index = static_cast<int>(cache_.size()) - 1;
     while (start_index > 0 &&

test/msg_cache_unittest.cpp

@@ -78,7 +78,12 @@ void fillCacheEasy(message_filters::Cache<Msg> & cache, unsigned int start, unsi
 TEST(Cache, emptySurroundingInterval)
 {
   message_filters::Cache<Msg> cache(10);
-  const std::vector<std::shared_ptr<Msg const> > interval_data = cache.getSurroundingInterval(rclcpp::Time(5, 0), rclcpp::Time(9, 0));
+  const std::vector<std::shared_ptr<Msg const>> interval_data = cache.getSurroundingInterval(
+    rclcpp::Time(
+      5,
+      0), rclcpp::Time(
+      9, 0));
+
   EXPECT_EQ(interval_data.size(), (unsigned int) 0); // empty cache shall return empty interval
 }