Fix SAFE_ALLOCA to not exhaust the stack when in a loop.

Fix SAFE_ALLOCA to not exhaust the stack when in a loop.

Problem reported by Dmietry Antipov in thread leading to:
http://lists.gnu.org/archive/html/emacs-devel/2014-09/msg00713.html
This patch fixes only SAFE_ALLOCA, SAFE_NALLOCA, and SAFE_ALLOCA_LISP;
the experimental local_* macros enabled by USE_LOCAL_ALLOCATORS
remain unfixed.
* callproc.c (call_process): Save and restore sa_avail.
* lisp.h (USE_SAFE_ALLOCA): Define sa_avail.
(AVAIL_ALLOCA): New macro.
(SAFE_ALLOCA, SAFE_NALLOCA, SAFE_ALLOCA_LISP):
Use it, and check against sa_avail rather than MAX_ALLOCA.