Check unchecked iterator access

Author: Sploder12

src/daemon/daemon.cpp

@@ -3562,9 +3562,17 @@ mp::Daemon::async_wait_for_ssh_and_start_mounts_for(const std::string& name, con
     try
     {
         auto it = operative_instances.find(name);
+        if (it == operative_instances.end())
+        {
+            // Level error since this can only occur because of a race condition or programming error.
+            auto msg = fmt::format("Instance '{}' cannot be waited on since it does not exist.\n", name);
+            mpl::log(mpl::Level::error, category, msg);
+            fmt::format_to(std::back_inserter(errors), msg);
+            return fmt::to_string(errors);
+        }
+
         auto vm = it->second;
 
-        // unchecked iterator access :)
         if (vm->current_state() == VirtualMachine::State::unavailable)
         {
             return fmt::to_string(errors);