patch 8.2.0571: double free when passing invalid argument to job_start()

Problem:    Double free when passing invalid argument to job_start().
Solution:   Clear the argument when freed. (Masato Nishihata, closes #5926)
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index 11f3362..af809ec 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -1681,6 +1681,7 @@
   call assert_fails('let job = job_start(["   "])', 'E474:')
   call assert_fails('let job = job_start("")', 'E474:')
   call assert_fails('let job = job_start("   ")', 'E474:')
+  call assert_fails('let job = job_start(["ls", []])', 'E730:')
   %bw!
 endfunc