Double free with latest updates

[Vin Shelton <acs@alumni.princeton.edu>]

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The most recent changes to builtin.c result in a double free in running the
test suite.

I have more details if you need them.

  - Vin

zsh
remote: Counting objects: 9, done.
remote: Compressing objects: 100% (5/5), done.
remote: Total 5 (delta 4), reused 0 (delta 0)
Unpacking objects: 100% (5/5), done.
>From git://zsh.git.sf.net/gitroot/zsh/zsh
   5b00bfe..1e6fb1a  master     -> origin/master
Updating 5b00bfe..1e6fb1a
Fast-forward
 ChangeLog     | 4 ++++
 Src/builtin.c | 7 ++++---
 2 files changed, 8 insertions(+), 3 deletions(-)

Building zsh
/opt/src/zsh-2015-05-04
cd . && ./.preconfig
Configuring zsh-2015-05-04 for installation prefix /opt/zsh-2015-05-04.
Building zsh...done.
Running tests...%
                print ten ten nine one print

  print print one two three four five six seven eight nine ten one two

  print mystery sequence

  print one

  print two

  print mystery sequence

  print metaphor\? shmetaphor!

  print metaphor!

  print -l metophor, Molochi,

  echo $(echo foo bar) again

  echo more $( echo $(echo foo bar) again )

done.

*** Error in `zsh': double free or corruption (out): 0x00007fff694af880 ***