r20570: use talloc_asprintf() instead of talloc_asprintf_append()

r20570: use talloc_asprintf() instead of talloc_asprintf_append()
because talloc_asprintf_append() doesn't work like talloc_append_string()
which uses strlen() on the old string instead of (tc->size - 1)

This matter in this case because strlower_talloc() over allocates

tridge: how should we fix this in lib/talloc/?

metze
(This used to be commit 1748af20b1f7b325048619a1e66a1c8b69474c5f)