Fix crasher in array slicing.
Fix crasher in array slicing.

Summary: `count - 1` is erroneously used as a bound on the length of a subarray based on the length of an array. This would mean a zero-length array would underflow and crashes would ensue.

Reviewed By: migchez

Differential Revision: D3561241

fbshipit-source-id: b647b3bbeb0f3ad671bc09a6bf4fc8ba8238cb36

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