cmd/pprof: index out of bounds crash [1.13 backport]

[gopherbot]

@dmitshur requested issue #33802 to be considered for backport to the next 1.13 minor release.

@gopherbot, please backport to 1.13.

[gopherbot]

Change https://golang.org/cl/191737 mentions this issue: [release-branch.go1.13] cmd/vendor: update vendored x/arch repo to 8a70ba74b3a1

[gopherbot]

Closed by merging 633d0c9 to release-branch.go1.13.