crypto/hmac: 1.15 change in behaviour of the Reset method [1.15 backport]

[gopherbot]

@FiloSottile requested issue #41089 to be considered for backport to the next 1.15 minor release.

Thank you for the report, this should absolutely work and is a regression. @gopherbot, open a Go 1.15 backport issue, please.

[FiloSottile]

False alarm, the report is about an invalid use of the code.