reflect: use Value.Len instead of conversion to slice header

[nevkontakte]

This change is functionally equivalent, but reduces reliance on unsafe
features. This would allow GopherJS to avoid an additional patch to the
standard library we'd have to maintain in order to remain compatible
with Go 1.17+.

[gopherbot]

This PR (HEAD: d1ac000) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/go/+/349469 to see it.

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See the Wiki page for more info

[gopherbot]

Message from Go Bot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
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During May-July and Nov-Jan the Go project is in a code freeze, during which
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[gopherbot]

Message from Keith Randall:

Patch Set 1:

(1 comment)

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After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Alexander Zhirov:

Patch Set 1:

(1 comment)

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After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Keith Randall:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/349469.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR (HEAD: 94ebb39) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/go/+/349469 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[gopherbot]

Message from Alexander Zhirov:

Patch Set 2:

(1 comment)

---

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After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Keith Randall:

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

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After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Go Bot:

Patch Set 2:

(1 comment)

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After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Go Bot:

Patch Set 2: TryBot-Result+1

(1 comment)

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After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR is being closed because golang.org/cl/349469 has been merged.