[R] Fix segfault in Table to Array conversion

[asfimport]

See https://github.com/fsaintjacques/arrow/runs/564315427#step:6:2169

Reporter: Francois Saint-Jacques / @fsaintjacques
Assignee: Francois Saint-Jacques / @fsaintjacques

Related issues:

• [R][C++] Fix crashing test in test-dataset.R on 32-bit Windows from ARROW-7979 (relates to)

PRs and other links:

• GitHub Pull Request #6871

_{Note: This issue was originally created as ARROW-8354. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Looks like the crash happened in Table__to_dataframe in case it gives any clues

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 6871
#6871