mime: quadratic complexity in WordDecoder.DecodeHeader [1.26 backport]

[gopherbot]

@neild requested issue #79217 to be considered for backport to the next 1.26 minor release.

@gopherbot please open backport issues for this security issue.

[gopherbot]

Change https://go.dev/cl/774861 mentions this issue: [release-branch.go1.26] mime: avoid quadratic complexity in WordDecoder.DecodeHeader