Allow for null endptr

lib/libc/atoi.c

 /*
 ** Copyright 2001, Travis Geiselbrecht. All rights reserved.
 ** Distributed under the terms of the NewOS License.
 */
 /*
  * Copyright (c) 2008 Travis Geiselbrecht
  *
  * Permission is hereby granted, free of charge, to any person obtaining
  * a copy of this software and associated documentation files
  * (the "Software"), to deal in the Software without restriction,
@@ skipping 148 matching lines @@
 
         if (c >= 'A' && c <= 'Z') {
             v = c - 'A' + 10;
         } else if (c >= 'a' && c <= 'z') {
             v = c - 'a' + 10;
         } else if (c >= '0' && c <= '9') {
             v = c - '0';
         }
 
         if (v < 0 || v >= base) {
-            *endptr = (char *) nptr;
+            if (endptr) {
+                *endptr = (char *) nptr;
+            }
             break;
         }
 
         new_ret = ret * base;
         if (new_ret / base != ret ||
                 new_ret + v < new_ret ||
                 ret == ULONG_MAX) {
             ret = ULONG_MAX;
             errno = ERANGE;
         } else {
             ret = new_ret + v;
         }
 
         nptr++;
     }
 
     if (neg && ret != ULONG_MAX) {
         ret = -ret;
     }
 
     return ret;
 }