A literal array may hold a hole; static_variables may not (#260)

[EdmondDantes]

Closes #260.

op_array_emalloc_copy_array rebuilds two different tables and asserted an
invariant that holds for one of them.

Static variables need every bucket kept: BIND_LEXICAL reaches a captured
variable by a byte offset into arData, computed at compile time, so a rebuild
that dropped a slot would shift every offset past it. The compiler seeds every
slot there, and the assert is right — it just belonged to that caller.

A literal array carries no such promise. [0 => 'a', 2 => 'b'] compiles to a
packed hash whose slot 1 is UNDEF, so nNumUsed is 3 while the element count is
2, and every debug build aborted the moment a task closure held one:

php: ext/async/thread.c:2342: op_array_emalloc_copy_array:
Assertion `zend_hash_num_elements(src) == src->nNumUsed' failed.

The rebuild itself was already correct for that case — it iterates by key, and
the keys are what the VM reads back — so this moves the assert to the
static_variables branch of op_array_to_emalloc and leaves the literal path
alone.

Reproduction

Twelve lines, no server involved:

spawn(function() {
    $pool = new ThreadPool(1);
    $future = $pool->submit(function () {
        $sparse = [0 => 'a', 2 => 'b'];
        return implode(',', $sparse);
    });
    echo await($future), "\n";
    $pool->close();
});

Aborts on main, prints a,b with this patch. It is tests/thread_pool/103-task_sparse_literal_array.phpt, which also covers a nested sparse array and string keys beside it.

What it was breaking

Every debug CI job of true-async/server since the merges of #254/#255/#256
earlier today, through tests/phpt/websocket/054-topics-pool-unsubscribe.phpt —
a pool worker loading a handler closure. Server commit 5392ff7 was green at
09:11 UTC and the same commit failed at 12:50 UTC with no change on its side.

ext/async/tests: 1104 passed, 0 failed. true-async/server tests/phpt
against this build: 390 passed, 0 failed, both on a debug ZTS build.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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