reflect: panic from malloc after MakeFunc function returns value that is also stored globally [1.13 backport]

[gopherbot]

@randall77 requested issue #39541 to be considered for backport to the next 1.13 minor release.

@gopherbot please open backport issues for 1.14 and 1.13.

[randall77]

This issue is pretty rare, but causes random GC crashes when it does happen. Seems worth backporting.

[gopherbot]

Change https://golang.org/cl/238862 mentions this issue: [release-branch.go1.13] reflect: zero stack slots before writing to them with write barriers

[gopherbot]

Closed by merging 3c1722c to release-branch.go1.13.