libotutil/checksum-utils: Fix memory management

libotutil/checksum-utils: Fix memory management

Ostree uses the OtChecksum data structure as a wrapper around GChecksum
(depending on what libraries are available at compile time). According
to the docs for g_checksum_get_digest(), a GChecksum value can no longer
be updated after that function is called. Ostree enforces this by
setting "initialized" to FALSE after getting the digest, but this leads
to ot_checksum_clear() avoiding freeing any memory, leading to leaks. So
this commit adds a "closed" value that gets set when getting a digest
and checked when updating the value, so the initialized value can be
used only for memory management.

Closes: #1521
Approved by: jlebon