NetEq: Align correlation buffer size in Merge with DspHelper expectations

The peak-detection algorithm (`DspHelper::PeakDetection`) expects one
extra element in the input buffer when `num_peaks == 1` because the
boundary parabolic fit algorithm may read index `data_length`.

While `TimeStretch` allocated this extra element, `Merge` did not, which
constituted a minor API contract mismatch. Although this is mathematically
unreachable in practice due to zero-initialization constraints in the
correlation pipeline, we should align `Merge`'s allocation with
`DspHelper`'s expectations to improve code consistency and readability.

This CL adds the `+1` safety margin to the correlation buffer allocation
in `Merge` to match the pattern used in `TimeStretch`. We also modernize
the allocation to use std::vector for cleaner memory management.

BUG=None

Change-Id: Ie932dd298892bfba3b7c35f34e1774e13ecee34f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/475001
Auto-Submit: Henrik Lundin <henrik.lundin@webrtc.org>
Reviewed-by: Jakob Ivarsson‎ <jakobi@webrtc.org>
Commit-Queue: Jakob Ivarsson‎ <jakobi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47821}