Fix crash when saving cache

[marcelofabri]

Fixes #2032

[SwiftLintBot]

	12 Messages
📖	Linting Aerial with this PR took 0.43s vs 0.41s on master (4% slower)
📖	Linting Alamofire with this PR took 3.46s vs 3.52s on master (1% faster)
📖	Linting Firefox with this PR took 14.1s vs 14.06s on master (0% slower)
📖	Linting Kickstarter with this PR took 21.29s vs 21.72s on master (1% faster)
📖	Linting Moya with this PR took 1.98s vs 2.05s on master (3% faster)
📖	Linting Nimble with this PR took 1.97s vs 1.99s on master (1% faster)
📖	Linting Quick with this PR took 0.6s vs 0.59s on master (1% slower)
📖	Linting Realm with this PR took 3.82s vs 3.9s on master (2% faster)
📖	Linting SourceKitten with this PR took 1.24s vs 1.22s on master (1% slower)
📖	Linting Sourcery with this PR took 5.13s vs 5.17s on master (0% faster)
📖	Linting Swift with this PR took 17.06s vs 17.4s on master (1% faster)
📖	Linting WordPress with this PR took 17.17s vs 17.38s on master (1% faster)

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c87b2c7). Click here to learn what that means.
The diff coverage is 80.95%.

@@           Coverage Diff            @@
##             master   #2109   +/-   ##
========================================
  Coverage          ?   89.8%           
========================================
  Files             ?     264           
  Lines             ?   15275           
  Branches          ?     997           
========================================
  Hits              ?   13717           
  Misses            ?    1539           
  Partials          ?      19

Impacted Files	Coverage Δ
...sts/SwiftLintFrameworkTests/LinterCacheTests.swift	98.62% <100%> (ø)
Source/SwiftLintFramework/Models/LinterCache.swift	80.26% <60%> (ø)

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