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

@@ -1894,6 +1894,14 @@ 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]);
 
 	if (job != NULL)
@@ -1901,6 +1909,7 @@ f_job_status(typval_T *argvars, typval_T *rettv)
 	    rettv->v_type = VAR_STRING;
 	    rettv->vval.v_string = vim_strsave((char_u *)job_status(job));
 	}
+    }
 }
 
 /*

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,
 /**/