Avoid access of possible dangling driver references. Fixes #978.

This is a two-step fix: destroy() is used instead of delete for the event driver and the driver core, so that there can not be any dangling references left. Additionally, the Timer struct now only tries to release the timer counter when the library has not yet been shut down. This avoids triggering an assertion, which in turn results in the InvalidMemoryOperationError in #978.

source/vibe/core/core.d

@@ -726,7 +726,7 @@ struct Timer {
 	~this()
 	{
 		debug assert(m_magicNumber == 0x4d34f916);
-		if (m_driver) m_driver.releaseTimer(m_id);
+		if (m_driver && s_core) m_driver.releaseTimer(m_id);
 	}
 
 	/// True if the timer is yet to fire.
@@ -1309,7 +1309,8 @@ shared static ~this()
 	if (!s_yieldedTasks.empty) tasks_left = true;
 	if (tasks_left) logWarn("There are still tasks running at exit.");
 
-	delete s_core;
+	destroy(s_core);
+	s_core = null;
 }
 
 // per thread setup
@@ -1319,7 +1320,7 @@ static this()
 	// object.Exception@src/rt/minfo.d(162): Aborting: Cycle detected between modules with ctors/dtors:
 	// vibe.core.core -> vibe.core.drivers.native -> vibe.core.drivers.libasync -> vibe.core.core
 	if (Thread.getThis().isDaemon && Thread.getThis().name == "CmdProcessor") return;
-			
+
 	assert(s_core !is null);
 
 	auto thisthr = Thread.getThis();

source/vibe/core/driver.d

@@ -46,7 +46,7 @@ package void setupEventDriver(DriverCore core_)
 package void deleteEventDriver()
 {
 	s_driver.dispose();
-	delete s_driver;
+	destroy(s_driver);
 	s_driver = null;
 }