Fix repl single step command cause exit and no jump_frame

[Katzeee]

As #898 says, when I am debuggin a single thread cpp program, call -exec n with cpptools or n with codelldb makes program suddenly exit.
I found auto_continue_if_many_stopped is set for multi-thread debugging, you don't want to jump to the stopped.threadId if we now hold a stopped_thread, but the jumping is reasonable if this StoppedEvent occurs at the thread we are now hold.

[mfussenegger]

Thanks for the report and PR. I wanted to add a test case and change the comment slightly but couldn't push on the PR. So I merged this via #901 instead