libostree: Add missing checks for invalid timestamps

libostree: Add missing checks for invalid timestamps

g_date_time_new_from_unix_utc() will not always return a valid GDateTime
— if the input timestamp is too big, GDateTime cannot represent it, and
the constructor returns NULL.

Add some missing checks for these situations. We don’t ever expect
timestamps to be this big, but they could be as a result of corruption
or a malicious repository.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #825
Approved by: cgwalters