[C++][Parquet] Avoid misaligned stores when reading BYTE_ARRAY decimals

[wgtmac]

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

Describe the bug

Reading BYTE_ARRAY-backed Decimal32 values can trigger UBSan:

runtime error: store to misaligned address for type 'uint64_t'

The converter always writes two uint64_t values, even when the destination is Decimal32 (4 bytes) or Decimal64 (8 bytes):

arrow/cpp/src/parquet/arrow/reader_internal.cc

Lines 728 to 730 in d5eb88e

	auto out_ptr_view = reinterpret_cast<uint64_t*>(out_ptr);
	out_ptr_view[0] = 0;
	out_ptr_view[1] = 0;

This became reachable for Decimal32/64 after GH-44345 / PR #47427.

The stores should be replaced with:

std::memset(out_ptr, 0, type_length);

The existing TestReadDecimals.Decimal32ByteArray test reproduces it when alignment sanitization is enabled.

Component(s)

C++, Parquet

[HuaHuaY]

Issue resolved by pull request 51136
#51136