patch 8.0.0588: job_stop() often assumes the channel will be closed

Problem:    job_stop() often assumes the channel will be closed, while the job
            may not actually be stopped. (Martin Gammelsæter)
Solution:   Only assume the job stops on "kill".  Don't send a signal if the
            job has already ended. (closes #1632)

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    588,
 /**/
     587,
 /**/