bug#35576: 27.0.50; Emacs crash when reads an integer with radix > 36

[Tino Calancha]

emacs -Q:
;; Emacs crash when you eval the following form
M-: #37r1

;; Expected: you get the error:
;; Invalid read syntax: "integer, radix 37"



In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.11)
 of 2019-05-05
Windowing system distributor 'The X.Org Foundation', version 11.0.11902000
System Description: Debian GNU/Linux 9 (stretch)


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commit c5ffba787a10f80d17a0ebc7fc7e1fb0f754843d
Author: Tino Calancha <tino.calancha@gmail.com>
Date:   Sun May 5 20:24:03 2019 +0900

    src/lread.c (read_integer): Prevent from accessing a null buffer

diff --git a/src/lread.c b/src/lread.c
index 1c97805ca7..810e24d614 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -2660,19 +2660,17 @@ read_integer (Lisp_Object readcharfun, EMACS_INT radix)
      Also, room for invalid syntax diagnostic.  */
   size_t len = max (1 + 1 + UINTMAX_WIDTH + 1,
                    sizeof "integer, radix " + INT_STRLEN_BOUND (EMACS_INT));
-  char *buf = NULL;
+  char *buf = xmalloc (len);
   char *p = buf;
   int valid = -1; /* 1 if valid, 0 if not, -1 if incomplete.  */
 
   ptrdiff_t count = SPECPDL_INDEX ();
-
   if (radix < 2 || radix > 36)
     valid = 0;
   else
     {
       int c, digit;
 
-      buf = xmalloc (len);
       record_unwind_protect_ptr (free_contents, &buf);
       p = buf;
 
@@ -2718,8 +2716,10 @@ read_integer (Lisp_Object readcharfun, EMACS_INT radix)
 
   if (valid != 1)
     {
-      sprintf (buf, "integer, radix %"pI"d", radix);
-      invalid_syntax (buf);
+      xfree (buf);
+      char str[len];
+      sprintf (str, "integer, radix %"pI"d", radix);
+      invalid_syntax (str);
     }
 
   *p = '\0';

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