cpu_x86.c: avoid NULL-deref for invalid arguments

* src/cpu/cpu_x86.c (x86Decode): Do not dereference NULL
when "models" is NULL and nmodels is 1 or greater.

src/cpu/cpu_x86.c

 /*
  * cpu_x86.c: CPU driver for CPUs with x86 compatible CPUID instruction
  *
- * Copyright (C) 2009 Red Hat, Inc.
+ * Copyright (C) 2009-2010 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -954,6 +954,9 @@ x86Decode(virCPUDefPtr cpu,
     if (data == NULL || (map = x86LoadMap()) == NULL)
         return -1;
 
+    if (models == NULL && nmodels != 0)
+        return -1;
+
     candidate = map->models;
     while (candidate != NULL) {
         bool allowed = (models == NULL);