net/http: panic due to racy read of persistConn after handler panic [1.16 backport]

[gopherbot]

@FiloSottile requested issue #46866 to be considered for backport to the next 1.16 minor release.

@gopherbot, please open backport issues for CL 333191 as a potential security fix.

[gopherbot]

Change https://golang.org/cl/338551 mentions this issue: [release-branch.go1.16] net/http/httputil: close incoming ReverseProxy request body

[dmitshur]

Approved as a fix for a security issue. This backport applies to both 1.16 (this issue) and 1.15 (#47473).

[gopherbot]

Closed by merging accf363 to release-branch.go1.16.