patch 8.2.3006: crash when echoing a value very early

Problem:    Crash when echoing a value very early. (Naruhiko Nishino)
Solution:   Do not use a NUL to truncate the message, make a copy.
            (closes #8388)

src/testdir/test_startup.vim

@@ -1301,4 +1301,18 @@ func Test_write_in_vimrc()
   call delete('Xvimrc')
 endfunc
 
+func Test_echo_true_in_cmd()
+  let lines =<< trim END
+      echo v:true
+      call writefile(['done'], 'Xresult')
+  END
+  call writefile(lines, 'Xscript')
+  if RunVim([], [], '--cmd "source Xscript" --c q')
+    call assert_equal(['done'], readfile('Xresult'))
+  endif
+  call delete('Xscript')
+  call delete('Xresult')
+
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab