moderately clean handling of allocated storage on error during partial

moderately clean handling of allocated storage on error during partial
construction, in spite of not having exception handling in the language.

	* krb5_decode.c (clean_krb5_authenticator, clean_krb5_ticket,
	clean_krb5_enc_tkt_part,clean_krb5_ap_req,
	clean_krb5_ap_rep_enc_part, clean_krb5_safe,
	clean_krb5_priv_enc_part, clean_krb5_cred_enc_part,
	clean_krb5_error): new static functions to free objects that may
	be partially constructed.
	(setup_buf_only, setup_no_tagnum, setup_no_length, setup): define
	in terms of each other to remove duplication, then add local
	variable error_cleanup to common declarations.
	(clean_return): new macro, uses error_cleanup on rep if possible
	and the allows the argument to be returned.
	(alloc_field, check_apptag, next_tag, begin_structure,
	get_field_body, get_field, get_lenfield_body, get_lenfield): use
	clean_return.
	(free_field): new macro to simplify the writing of clean_*.
	(clear_field): macro to clean up preparation of fields for later
	use by clean_* functions.
	(decode_krb5_authenticator, decode_krb5_ticket,
	decode_krb5_encryption_key, decode_krb5_enc_tkt_part,
	decode_krb5_enc_kdc_rep_part, decode_krb5_as_rep,
	decode_krb5_tgs_rep, decode_krb5_ap_req, decode_krb5_ap_rep,
	decode_krb5_ap_rep_enc_part, decode_krb5_as_req,
	decode_krb5_tgs_req, decode_krb5_kdc_req_body, decode_krb5_safe,
	decode_krb5_priv, decode_krb5_enc_priv_part, decode_krb5_cred,
	decode_krb5_enc_cred_part, decode_krb5_error,
	decode_krb5_authdata, decode_krb5_pwd_sequence,
	decode_krb5_pwd_data, decode_krb5_padata_sequence,
	decode_krb5_alt_method, decode_krb5_etype_info): change setup
	macro to pass a cleanup method (or just free if there were no
	partial allocations, or 0 for the two cases with no allocation at
	all.) Also explicitly zero pointer subfields, since calloc is not
	a safe way to assure that. Generally, provide for automatic
	deallocation of storage on error.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@6785 dc483132-0cff-0310-8789-dd5450dbe970