[C++] Out-of-bounds buffer slice reading a Feather V1 file with inconsistent column metadata

When reading a Feather V1 file, `ReaderV1::LoadValues` in `cpp/src/arrow/ipc/feather.cc` reads `total_bytes` of data for a column, then carves null-bitmap, offsets, and values sub-buffers using sizes derived from `meta->length()`. That length comes from the footer flatbuffer (parsed with `GetCTable` and never verified) and is independent of the data buffer, so a file declaring a large `length` but a small `total_bytes` produces `SliceBuffer` calls that fall outside the backing buffer, and the trailing `buffer->size() - offset` underflows. The resulting oversized buffer views lead to an out-of-bounds read when the array is consumed.

`SliceBuffer` only validates in debug builds, so the slices should use `SliceBufferSafe` and reject negative `length`/`null_count`.