More debug allocator fixes and enhancements

More debug allocator fixes and enhancements

src/alloc-srvx.c (*_MAGIC): ASk recognized the CCSDS ASM sequence.
(srvx_free): Overwrite with 0xDE bytes to distinguish from
uninitialized values.  As SailorFrag suggested, only overwrite the
user region.

src/dict-splay.c (dict_insert): Check whether free functions need to
be worked around here as well.
git-archimport-id: srvx@srvx.net--2005-srvx/srvx--devo--1.3--patch-6