[Rust] "allocate_aligned"/"reallocate" need to initialize memory to avoid UB

[asfimport]

See this thread for background.

Reporter: Paddy Horan / @paddyhoran
Assignee: Paddy Horan / @paddyhoran

PRs and other links:

• GitHub Pull Request #6422

_{Note: This issue was originally created as ARROW-7836. Please see the migration documentation for further details.}

[asfimport]

Paddy Horan / @paddyhoran:
Issue resolved by pull request 6422
#6422