Avoid signed integer overflow in substr()

Perform negation after the (size_t) cast rather than before,
so as to avoid a signed integer overflow for PHP_INT_MIN.

Fixes oss-fuzz #

Avoid signed integer overflow in substr()

Perform negation after the (size_t) cast rather than before,
so as to avoid a signed integer overflow for PHP_INT_MIN.

Fixes oss-fuzz #31069.

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