r4136: when we have a size or switch variable that is a pointer we need to check...

r4136: when we have a size or switch variable that is a pointer we need to check that the server
hasn't given us a null pointer, otherwise we can segv when we dereference it. For example:

  [size_is(*size)] *x;
  uint32 *size;

if a broken server gave us x != NULL and size == NULL then we would crash. I've added
a check_null_pointer() call in pidl to catch this.
(This used to be commit 8cbd3f47a95367e861c6b99c44416a9ccef3c9ca)