patch 8.2.3379: crash when using NULL job

Problem:    Crash when using NULL job.
Solution:   Copy static string into buffer. (issue #8260)
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index 783a41d..fa510c7 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -1718,6 +1718,10 @@
   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 = {}