Little conveniences

[ghudson@MIT.EDU]

As part of the encryption performance work, I'm finding myself dearly
wanting a couple of internal conveniences.  So, these are likely to
hit k5-int.h in the trunk in the next few weeks.  First:

  static inline void zapfree(void *ptr, size_t len)
  {
      if (ptr) {
          zap(ptr, len);
          free(ptr);
      }
  }

This builds on the existing zap() macro or function.  Particularly in
the crypto code, it avoids the need for a lot of repeated four-line
cleanups.

Second:

  /* Allocate zeroed memory; set *code to 0 on success or ENOMEM on failure. */
  static inline void *k5alloc(size_t size, krb5_error_code *code)
  {
      void *ptr;

      ptr = calloc(size, 1);
      *code = (ptr == NULL) ? ENOMEM : 0;
      return ptr;
  }

This is to save three lines in the (very common) allocate/check/error
pattern used all over the code.  With k5alloc that can be three lines
instead of five:

  ptr = k5alloc(size, &ret);
  if (!ptr)
      goto cleanup;

The downside is that everyone knows what malloc() does, but not
everyone will immediately know what k5alloc does.  It also uses
krb5_error_code in a funny way, since the more common construction
("ret = k5alloc(size, &ptr);") can't be written as a C function due to
the way C void pointers work.  So I'm not 100% pleased with this one.
But the alternative is to spend an awful lot of code space when
allocating multiple chunks of memory, as is commonly done in the
crypto code.