patch 8.2.3379: crash when using NULL job

Problem:    Crash when using NULL job.
Solution:   Copy static string into buffer. (issue #8260)

src/job.c

@@ -1983,7 +1983,10 @@ job_to_string_buf(typval_T *varp, char_u *buf)
     char  *status;
 
     if (job == NULL)
-	return (char_u *)"no process";
+    {
+	vim_snprintf((char *)buf, NUMBUFLEN, "no process");
+	return buf;
+    }
     status = job->jv_status == JOB_FAILED ? "fail"
 		    : job->jv_status >= JOB_ENDED ? "dead"
 		    : "run";

src/testdir/test_channel.vim

@@ -1718,6 +1718,10 @@ func Test_job_stop_immediately()
   endtry
 endfunc
 
+func Test_null_job_eval()
+  call assert_fails('eval test_null_job()->eval()', 'E121:')
+endfunc
+
 " This was leaking memory.
 func Test_partial_in_channel_cycle()
   let d = {}

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3379,
 /**/
     3378,
 /**/