bug#8318: int overflow problem in SAFE_ALLOCA, SAFE_ALLOCA_LISP

[Paul Eggert]

The SAFE_ALLOCA macro assumes that adding 1 to the integer
variable sa_must_free cannot overflow, but this assumption
is incorrect in some cases.  I plan to commit the following
patch to fix this.

I found this bug using gcc 4.5.2 -O2 -Wstrict-overflow.

* lisp.h (SAFE_ALLOCA, SAFE_ALLOCA_LISP): Avoid 'int' overflow
leading to a memory leak, possible in functions like
load_charset_map_from_file that can allocate an unbounded number
of objects.
=== modified file 'src/lisp.h'
--- src/lisp.h  2011-03-18 04:58:44 +0000
+++ src/lisp.h  2011-03-22 09:04:53 +0000
@@ -3602,7 +3602,7 @@
     else                                                 \
       {                                                          \
        buf = (type) xmalloc (size);                      \
-       sa_must_free++;                                   \
+       sa_must_free = 1;                                 \
        record_unwind_protect (safe_alloca_unwind,        \
                               make_save_value (buf, 0)); \
       }                                                          \
@@ -3632,7 +3632,7 @@
        buf = (Lisp_Object *) xmalloc (size_);            \
        arg_ = make_save_value (buf, nelt);               \
        XSAVE_VALUE (arg_)->dogc = 1;                     \
-       sa_must_free++;                                   \
+       sa_must_free = 1;                                 \
        record_unwind_protect (safe_alloca_unwind, arg_); \
       }                                                          \
   } while (0)