Fix error in __vsnprintf_chk()

__vsnprintf_chk() passed the wrong length argument to the vsnprintf()
call. I'm not aware of any specific bug this solves, but I found this
error while auditing the *_chk() functions to figure out why "rogue"
works when compiled with -DUSE_FORTIFY_LEVEL=1 but not with
USE_FORTIFY_LEVEL=2.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

libc/stdio/__vfprintf_chk.c

@@ -24,7 +24,7 @@ int __vsnprintf_chk(char *s, size_t maxlen, int flags, size_t slen,
     if (slen < maxlen) {
         abort();
     }
-    return vsnprintf(s, slen, format, args);
+    return vsnprintf(s, maxlen, format, args);
 }
 
 int __sprintf_chk(char *s, int flags, size_t slen, const char *format, ...)