…9288)

# Which issue does this PR close?

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- Closes apache#9286

# Rationale for this change

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Miri reports undefined behaviour because `truncate` didn't check new
length when setting the length for `BufferBuilder`, meaning when used in
conjunction with `as_slice_mut` the pointer could potentially be
dereferenced for memory beyond its allocation.

# What changes are included in this PR?

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Ensure we bound length in BufferBuilder so truncating to a larger length
does not change the length.

# Are these changes tested?

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Added test.

# Are there any user-facing changes?

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No.