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)
diff --git a/src/version.c b/src/version.c
index 7aae749..3b84b05 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    588,
+/**/
     587,
 /**/
     586,