Performance/CacheValueOverride: add some more defensive coding #371

Job	Run time
QTest + Lint: PHP latest - PHPCS stable	17s
QTest: PHP 5.4 - PHPCS lowest	1m 9s
QTest + Lint: PHP 5.4 - PHPCS stable	1m 0s
QTest: PHP latest - PHPCS lowest	20s
	2m 46s

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