Durations don't always zero init

In a couple of places I assumed that durations and time_points always
zero-initialised. This is incorrect - they act as if they were their
representation, so integer durations only zero-init when static.

You can zero init (like integers) by having `{}` initialisation.

drivers/Ticker.h

@@ -149,7 +149,7 @@ protected:
     void attach_absolute(Callback<void()> func, TickerDataClock::time_point abs_time);
 
     void handler() override;
-    std::chrono::microseconds  _delay;  /**< Time delay (in microseconds) for resetting the multishot callback. */
+    std::chrono::microseconds  _delay{0};  /**< Time delay (in microseconds) for resetting the multishot callback. */
     Callback<void()>    _function;  /**< Callback. */
     bool          _lock_deepsleep;  /**< Flag which indicates if deep sleep should be disabled. */
 #endif

drivers/Timer.h

@@ -111,8 +111,8 @@ protected:
     TimerBase(const ticker_data_t *data, bool lock_deepsleep);
     ~TimerBase();
     std::chrono::microseconds slicetime() const;
-    TickerDataClock::time_point _start;   // the start time of the latest slice
+    TickerDataClock::time_point _start{};   // the start time of the latest slice
-    std::chrono::microseconds _time;    // any accumulated time from previous slices
+    std::chrono::microseconds _time{};    // any accumulated time from previous slices
     TickerDataClock _ticker_data;
     bool _lock_deepsleep;    // flag that indicates if deep sleep should be disabled
     bool _running = false;   // whether the timer is running

drivers/source/Timer.cpp

@@ -100,7 +100,7 @@ microseconds TimerBase::elapsed_time() const
 microseconds TimerBase::slicetime() const
 {
     CriticalSectionLock lock;
-    microseconds ret;
+    microseconds ret = 0us;
     if (_running) {
         ret = _ticker_data.now() - _start;
     }