Check that the Timestamp String is valid

The current `FormatISO8601DateTime` function will
return an empty string if it encounters an error
when converting the `int64_t` seconds since epoch
to a formatted date time. In the unlikely case that happens,
`strStamped.pop_back()` would be undefined behavior.

src/logging.cpp

@@ -352,7 +352,7 @@ std::string BCLog::Logger::LogTimestampStr(const std::string& str)
         const auto now{SystemClock::now()};
         const auto now_seconds{std::chrono::time_point_cast<std::chrono::seconds>(now)};
         strStamped = FormatISO8601DateTime(TicksSinceEpoch<std::chrono::seconds>(now_seconds));
-        if (m_log_time_micros) {
+        if (m_log_time_micros && !strStamped.empty()) {
             strStamped.pop_back();
             strStamped += strprintf(".%06dZ", Ticks<std::chrono::microseconds>(now - now_seconds));
         }