Fix disconnect race crash. (#147)

It was possible for the Target object to be destructed before it
stopped.
This manifested as a crash when freed memory was accessed.
Add a flag to wait and check.

include/FurbleControl.h

@@ -50,6 +50,8 @@ class Control {
    protected:
     Target(Camera *camera);
 
+    volatile bool m_Stopped = false;
+
    private:
     static constexpr UBaseType_t m_QueueLength = 8;
 

src/FurbleControl.cpp

@@ -75,6 +75,7 @@ void Control::Target::task(void) {
     }
   }
 task_exit:
+  m_Stopped = true;
   vTaskDelete(NULL);
 }
 
@@ -212,10 +213,18 @@ void Control::disconnect(void) {
 
   const std::lock_guard<std::mutex> lock(m_Mutex);
 
+  // send disconnect
   for (const auto &target : m_Targets) {
     target->sendCommand(CMD_DISCONNECT);
   }
 
+  // wait for tasks to finish
+  for (const auto &target : m_Targets) {
+    do {
+      vTaskDelay(pdMS_TO_TICKS(1));
+    } while (!target.get()->m_Stopped);
+  }
+
   m_Targets.clear();
   m_State = STATE_IDLE;
 }