Fix use after free in heredoc variables

[mwgamera]

After glibc upgrade (I think) one of my scripts started to complain about null variable in here document. The following may or may not reproduce the problem for you, but sealcountedbuffer(buf) frees its argument so using buf->len afterwards is illegal.

; cat a
{} << X
$a
X
; es a
a:3: null variable name in here document

[wryun]

Thanks, missed this.