runtime/pprof: TestLabelSystemstack due to sample with no location [1.19 backport]

[gopherbot]

@prattmic requested issue #51550 to be considered for backport to the next 1.19 minor release.

@gopherbot Please backport to 1.19. This is a fix for a flaky test, which is also flaky on 1.19.

[gopherbot]

Change https://go.dev/cl/474618 mentions this issue: [release-branch.go1.19] runtime/pprof: improve output of TestLabelSystemstack

[gopherbot]

Closed by merging 3a50af1 to release-branch.go1.19.