dbus: stop daemon when last instance detached

+ dbusinstance: count_ can be initialized to something else than 0
causing count_ to always be incorrect
+ ring_api: check pointer for ioContext() (because a callback may
be emitted right after ioContext() is resetted causing to end
on a segfault instead exit=0)

GitLab: #861
Change-Id: If0d798aaa2ce7d65426d902868627bd701652375

bin/dbus/dbusinstance.hpp

@@ -60,5 +60,5 @@ public:
     }
 
 private:
-    int_least16_t count_;
+    int_least16_t count_ {0};
 };

src/ring_api.cpp

@@ -125,7 +125,8 @@ logging(const std::string& whom, const std::string& action) noexcept
 void
 CallbackWrapperBase::post(std::function<void()> cb)
 {
-    jami::Manager::instance().ioContext()->post(std::move(cb));
+    if (auto io = jami::Manager::instance().ioContext())
+        io->post(std::move(cb));
 }
 
 } // namespace libjami