patch 8.2.1857: Vim9: using job_status() on an unused var gives an error

Problem:    Vim9: using job_status() on an unused var gives an error.
Solution:   Return "fail". (closes #7158)

src/job.c

@@ -1893,6 +1893,14 @@ f_job_start(typval_T *argvars, typval_T *rettv)
  */
     void
 f_job_status(typval_T *argvars, typval_T *rettv)
+{
+    if (argvars[0].v_type == VAR_JOB && argvars[0].vval.v_job == NULL)
+    {
+	// A job that never started returns "fail".
+	rettv->v_type = VAR_STRING;
+	rettv->vval.v_string = vim_strsave((char_u *)"fail");
+    }
+    else
     {
 	job_T	*job = get_job_arg(&argvars[0]);
 
@@ -1902,6 +1910,7 @@ f_job_status(typval_T *argvars, typval_T *rettv)
 	    rettv->vval.v_string = vim_strsave((char_u *)job_status(job));
 	}
     }
+}
 
 /*
  * "job_stop()" function

src/testdir/test_vim9_assign.vim

@@ -74,7 +74,11 @@ def Test_assignment()
 
   if has('channel')
     var chan1: channel
+    assert_equal('fail', ch_status(chan1))
+
     var job1: job
+    assert_equal('fail', job_status(job1))
+
     # calling job_start() is in test_vim9_fails.vim, it causes leak reports
   endif
   if has('float')

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1857,
 /**/
     1856,
 /**/