crypto/subtle: panic if slices of different lengths are passed to ConstantTimeCompare.
crypto/subtle: panic if slices of different lengths are passed to ConstantTimeCompare.

ConstantTimeCompare has always been documented to take equal length
slices but perhaps this is too subtle, even for 'subtle'.

Fixes issue 7304.

LGTM=hanwen, bradfitz
R=golang-codereviews, hanwen, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/62190043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang