delta: fix overflow when computing limit

When checking whether a delta base offset and length fit into the base
we have in memory already, we can trigger an overflow which breaks the
check. This would subsequently result in us reading memory from out of
bounds of the base.

The issue is easily fixed by checking for overflow when adding `off` and
`len`, thus guaranteeting that we are never indexing beyond `base_len`.
This corresponds to the git patch 8960844a7 (check patch_delta bounds
more carefully, 2006-04-07), which adds these overflow checks.

Reported-by: Riccardo Schirone <rschiron@redhat.com>