ethash: fix usage of extraNonce

5 jobs for master in 7 minutes and 22 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prepare
passed #53953087
dep

00:02:29

passed #53953086
libhash

00:01:51

 
  Build
passed #53953089
compile

00:02:39

passed #53953088
test

00:04:52

 
  External
passed continuous-integration/appveyor/branch