Backport GCPRO fix from trunk.

The bug was reported for AIX before today's changes.
I reproduced the problem on Fedora 17 x86-64 when setting
GC_MARK_STACK by hand, and I presume it occurs with default
configurations on HP-UX and Unixware.
Trunk fix on 2013-01-14 by Dmitry Antipov <dmantipov@yandex.ru>:
Fix compilation with GC_MARK_STACK == GC_USE_GCPROS_AS_BEFORE.
* eval.c (eval_sub): Protect `form' from being GCed before its
car and cdr becomes protected with the backtrace entry.

Fixes: debbugs:13650

1 files changed, 3 insertions, 0 deletions

diff --git a/src/eval.c b/src/eval.c
index a0db6e0745c..a21b2b05762 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -2034,7 +2034,10 @@ eval_sub (Lisp_Object form)
     return form;
 
   QUIT;
+
+  GCPRO1 (form);
   maybe_gc ();
+  UNGCPRO;
 
   if (++lisp_eval_depth > max_lisp_eval_depth)
     {