Fix unchecked malloc

[garrettgu10]

The original code did not check if the allocation succeeded, leading to possible issues if malloc() returns null.

This PR should fix that by simply replacing the malloc with a stack allocation.

[github-actions]

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[garrettgu10]

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[garrettgu10]

recheck

[jclee]

Just naively wondering... If the malloc is sometimes failing, is it just due to running out of heap space when trying to allocate a large block of memory? If so, wouldn't allocating on the stack just introduce a different failure mode where stack space is exhausted?

[kentonv]

@jclee The malloc is not ever failing. This was flagged by a security scanner tool. In practice we have overcommit enabled so mallocs never fail. But we also shouldn't be using malloc anywhere in KJ code.