Zero length fields when freeing object contents

Zero length fields when freeing object contents

In krb5_free_data_contents() and krb5_free_checksum_contents(), zero
the length as well as the data pointer to leave the object in a valid
state.  Add asserts to existing test harnesses to verify the new
behavior.

In the krb5 GSS mech's kg_checksum_channel_bindings(), remove the code
to reallocate the checksum with xmalloc(), as it relied on
krb5_free_checksum_contents() leaving the object in an invalid state.
This code was added in commit a30fb4c4400f13a2690df7ef910b7ac0ccbcf194
to match an xfree() call, but commit
29337e7c7b796685fb6a03466d32147e17aa2d16 replaced that xfree() with a
krb5_free_checksum_contents().  (In addition, the xmalloc and xfree
wrappers never evolved to do anything beyond malloc and free.)

In kpropd's recv_database(), don't free outbuf until we are done using
its length.

[ghudson@mit.edu: rewrote commit message; edited doxygen comment
changes to mention version]

ticket: 8871 (new)