[C++][JSON] Avoid reusing simdjson values after failed get() #55497

Describe the bug, including details regarding any error messages, version, and platform.

StructConverter::AppendValue() in cpp/src/arrow/json/from_string.cc probes json_obj.get(array), then reuses the same ondemand value as an object. simdjson documents that a value may be consumed after a failed get(), making further use undefined.

Observed on Linux x86_64 with simdjson 4.6.4 fallback implementation: Invalid: Error getting field of object: TAPE_ERROR

Minimal repro:

[
  [{"c11": null, "c12": {"c121": -1}}, 1],
  [{"c11": 2, "c12": {"c121": null}}, 2]
]

Parsing this as struct<c1: struct<c11: int32, c12: struct<c121: int32>>, c2: int64> fails.

Component(s)

C++