Fix various issues detected by static analysis

Fix various issues detected by static analysis

In klists's show_credential(), ensure that the column counter doesn't
decrease if printf() fails.

In process_k5beta7_princ(), bounds-check the e_length field.

In ndr_enc_delegation_info(), initialize b so it is always valid for
the cleanup handler.

In krb5_dbe_def_decrypt_key_data(), change the flow control so ret is
always set by the end of the function.  Return KRB5_KDB_INVALIDKEYSIZE
if there isn't enough data in the first key_data_contents field or if
the serialized key length is invalid.

In svcauth_gss_validate(), expand rpchdr to accomodate the header plus
MAX_AUTH_BYTES.

In svcudp_reply(), change slen to unsigned to match the return type of
XDR_GETPOS() and eliminate an unnecessary check for slen >= 0.

In krb5int_pthread_loaded()(), remove pthread_equal() from the weak
symbol checks.  It is implemented as an inline function in some glibc
versions, which makes the comparison "&pthread_equal == 0" always
false.

[ghudson@mit.edu: further modified krb5_dbe_def_decrypt_key_data() for
clarity; added detail to commit message]