Fix comparison of overflowing strings

Summary: Some clown introduced numeric overflow comparisons and forgot to negate a value in D1659152

Fixes #4303

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Reviewed By: @JoelMarcey

Differential Revision: D1697745

Signature: t1:1697745:1416607375:efec10b8e4762e2e1f387d1c723fa3aa4097374b