pool_reuse(): Zero-out the first block if zero_out==true
pool_reuse(): Zero-out the first block if zero_out==true

This is needed because pool_alloc() doesn't zero-out the first block in
case of reusing.

Fortunately, this bug had no effect in most cases of reusing, but
it could cause a crash while inspecting dict macros in disjuncts.
In particular, it doesn't affect parsing.

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