util: disable cache if we have no build-id and timestamp is zero

Timestamp can be zero for example when Flatpak is used. In this
case just disable the cache rather then segfaulting when
incompatible cache items are loaded.

V2: actually return false when mtime is 0.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit ea66bfda)