IORing: `blockingConsumeCompletions()` drops context on error

[glessard]

Issue exposed as a test case:

extension IORingTests {
  func testMinimalContextLoss() throws {
    var ring = try IORing(queueDepth: 4)
    _ = try ring.submit(
      linkedRequests: .close(FileDescriptor(rawValue: -1), context: 42)
    )
    var received: UInt64? = nil
    ring.blockingConsumeCompletions { completion, _, _ in
      received = completion?.context
    }
    XCTAssertEqual(received, 42)  // fails: received is nil
  }
}

The completion handler runs, but completion is nil so there is no relation to the initial request. If the close() operation had succeeded, completion would be non-nil and the context would be available.