Clean-up: suppress bogus scan-build warning

scan-build seems confused about the address passed to free() and
reports that chunk elements are passed with an offset of 8 bytes from
the address that was returned by calloc().

Move the inner-trace chunk of an element to the start of the element
wrapper-structure to silence this warning.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>