bytes: re-slice buffer to its previous length after call to grow()

[forfuncsake]

Fixes #25435

The added test fails without the re-slice and passes with it.

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Message from Ian Lance Taylor:

Patch Set 2: Code-Review-2

(2 comments)

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Message from Dave Russell:

Patch Set 2:

(2 comments)

Thanks for the review

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Message from Ian Lance Taylor:

Patch Set 2:

(2 comments)

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Message from Dave Russell:

Patch Set 2:

(2 comments)

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This PR (HEAD: 11f1921) has been imported to Gerrit for code review.

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Message from Brad Fitzpatrick:

Patch Set 4: Run-TryBot+1 Code-Review+2

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Patch Set 4:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=04b77217

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Patch Set 4: TryBot-Result+1

TryBots are happy.

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This PR is being closed because golang.org/cl/113495 has been merged.