fix crash when reading msg log (#166)

include/util/ILogEntry.h

@@ -9,7 +9,8 @@
 class ILogEntry
 {
   public:
-    virtual size_t size(void) const = 0;
+    virtual size_t size(void) const = 0;   // size of the log entry, including header
+    virtual size_t length(void) const = 0; // length of the payload (without header)
     virtual size_t serialize(std::function<size_t(const uint8_t *, size_t)> write) const = 0;
     virtual size_t deserialize(std::function<size_t(uint8_t *, size_t)> read) = 0;
     virtual ~ILogEntry() = default;

include/util/LogMessage.h

@@ -12,6 +12,8 @@ constexpr uint32_t messagePayloadSize = 233;
  * Note: this struct does have vtable pointers, i.e. sizeof(LogMessageHeader)-8 is the real data size
  */
 struct LogMessageHeader : public ILogEntry {
+    size_t length(void) const override { return _size; }
+
     uint16_t _size;
     time_t time;
     uint32_t from;