Fix startAfter when object is not present

[Alfus]

The iterator always seeks to the value >= the entry, so the marker should only be skipped when it actually exists. Otherwise the iterator is already pointing at the next value.

See https://github.com/johannesboyne/gofakes3/issues/87

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f7e9c41) 65.92% compared to head (b359e41) 65.94%.

Files	Patch %	Lines
backend/s3mem/backend.go	85.71%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #88 +/- ## ========================================== + Coverage 65.92% 65.94% +0.02% ========================================== Files 28 28 Lines 2691 2693 +2 ========================================== + Hits 1774 1776 +2 - Misses 650 651 +1 + Partials 267 266 -1 ```

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[sonarcloud[bot]]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud