fix: stack-use-after-scope variable ordering

[jmayclin]

Resolved issues:

#4354

Description of changes:

Co-authored-by: Lindsay Stewart <[email protected]>

When running with newer address sanitizers, ASAN detects a stack-use-after-scope variable ordering issue.

  This frame has 4 object(s):
    [32, 40) 'total_bytes' (line 422)
    [64, 88) 'new_bufs' (line 426)
    [128, 384) 'new_bufs_mem' (line 427) <== Memory access at offset 128 is inside this variable
    [448, 456) 'bytes_written' (line 433)

Sequence of events

1. s2n_blob new_bufs is defined
2. new_bufs_mem is defined
3. function finishes
4. new_bufs_mem goes out of scope first (because it was defined last)
5. s2n_blob new_bufs goes out of scope, triggering the DEFER_CLEANUP
6. this zeros the buffer of new_bufs, but that buffer (new_bufs_mem) is already out of scope

This commit switches the order of new_bufs and new_bufs_mem so that new_bufs_mem is still in scope when s2n_free_or_wipe accesses it.

Call-outs:

This failure does reproduce in #4048. build dashboard I cleaned up that PR and will be merging it after this fix is merged in.

Testing:

Confirmed on local host that ASAN errors no longer occur. I then rebased this commit on the ASAN commit branch and ran it under the new codebuild job to confirm that no errors were found.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[camshaft]

Looks good with Lindsay's change