flush buffer before early stream retrun in init_stream

[jubrad]

Motivation

We're seeing intermittent SSL SYSCALL error: EOF detected in our cloud tests... only in CI
After a lengthy and annoying git bisect we believe this is a balancerd change, #34452 seems like the only likely change.

I'm not positive why a flush is necessary when the no password branch on line 790, didn't have one and I believe would have been hit prior to this.

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.