fix(termdebug): handle exiting during startup properly (#16790)

s:EndTermDebug should only be called when exiting if the debugger started
without error, otherwise the plugin breaks.

Vim handles this by using job_setoptions to set the on_exit callback to
s:EndTermDebug after startup succeeds. However, Nvim does not have such
functionality; instead; use s:starting to mimic this behaviour.

Also, introduce s:running to fix s:CheckGdbRunning; it did not work correctly
due to the "[Process exited X]" message keeping the job's channel alive (though
the stream is closed). This means nvim_get_chan_info cannot be used to check if
the debugger has exited, as it may still return a non-empty dict.

runtime/pack/dist/opt/termdebug/plugin/termdebug.vim

@@ -197,7 +197,7 @@ func s:CloseBuffers()
 endfunc
 
 func s:CheckGdbRunning()
-  if nvim_get_chan_info(s:gdb_job_id) == {}
+  if !s:running
       echoerr string(s:GetCommand()[0]) . ' exited unexpectedly'
       call s:CloseBuffers()
       return ''
@@ -280,6 +280,8 @@ func s:StartDebug_term(dict)
     call s:CloseBuffers()
     return
   endif
+  let s:running = v:true
+  let s:starting = v:true
   let gdb_job_info = nvim_get_chan_info(s:gdb_job_id)
   let s:gdbbuf = gdb_job_info['buffer']
   let s:gdbwin = win_getid(winnr())
@@ -355,6 +357,8 @@ func s:StartDebug_term(dict)
     sleep 10m
   endwhile
 
+  let s:starting = v:false
+
   " Set the filetype, this can be used to add mappings.
   set filetype=termdebug
 
@@ -663,6 +667,11 @@ func s:GetAsmAddr(msg)
 endfunc
 
 function s:EndTermDebug(job_id, exit_code, event)
+  let s:running = v:false
+  if s:starting
+    return
+  endif
+
   if exists('#User#TermdebugStopPre')
     doauto <nomodeline> User TermdebugStopPre
   endif