Fix negative plot sync durations not crashing the harvester

[felixbrucker]

Purpose:

Ensure negative durations do not crash the harvester.

Current Behavior:

A negative duration will crash the harvester.

New Behavior:

A negative duration will not crash the harvester.

Side note:

I don't know why the bug report was closed without any comment half a year ago, but this issue was present since 1.3.4 and still is in the latest chia version as of writing. This occurring might be rare but it does happen and should not just be closed, especially when the fix is a one liner.

Fixes #15027

[coveralls-official]

Pull Request Test Coverage Report for Build 7735134136

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• 0 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• 764 unchanged lines in 50 files lost coverage.
• Overall coverage decreased (-0.05%) to 90.842%

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Files with Coverage Reduction	New Missed Lines	%
chia/daemon/server.py	1	87.31%
chia/data_layer/download_data.py	1	71.04%
chia/plotters/madmax.py	1	50.0%
chia/server/start_service.py	1	93.3%
chia/util/condition_tools.py	1	91.8%
chia/wallet/trade_manager.py	1	96.26%
chia/wallet/util/wallet_sync_utils.py	1	78.14%
tests/core/util/test_file_keyring_synchronization.py	1	97.1%
tests/farmer_harvester/test_farmer.py	1	99.68%
tests/fee_estimation/test_fee_estimation_unit_tests.py	1	99.06%

Totals
Change from base Build 7726492794:	-0.05%
Covered Lines:	96340
Relevant Lines:	106026

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💛 - Coveralls