net/http: panic on misformed If-None-Match Header with http.ServeContent [1.14 backport]

[gopherbot]

@networkimprov requested issue #39817 to be considered for backport to the next 1.14 minor release.

@gopherbot please backport to 1.14. CL at https://golang.org/cl/240343

[networkimprov]

@dmitshur this may also need a 1.13 backport.

cc @FiloSottile @bradfitz

[FiloSottile]

Opened #39925

[dmitshur]

Backport approved for Go 1.14 (this issue) and 1.13 (#39925) as this is a serious issue with no workaround.

[gopherbot]

Change https://golang.org/cl/240343 mentions this issue: [release-branch.go1.14] net/http: fix panic with If-None-Match value in http.ServeContent

[gopherbot]

Closed by merging de0e241 to release-branch.go1.14.