Hotfix: crash when --quantize-optimization-steps is set to 0 (#759)
Hotfix: crash when --quantize-optimization-steps is set to 0 (#759)

* fix problem if the optimization step is set to 0
* set the first error residual to 0
emjotde/amunmt,emjotde/amunn,marian-nmt/marian-train,emjotde/Marian,emjotde/amunmt,marian-nmt/marian-train,emjotde/amunmt,emjotde/amunn,emjotde/Marian,marian-nmt/marian-train,marian-nmt/marian-train,emjotde/amunn,marian-nmt/marian-train,emjotde/amunn,emjotde/amunmt