Fix up _CheckSecondTokenNotSlotSuffixed function.

Incremental OTA generation was failing since the
function _CheckSecondTokenNotSlotSuffixed was in
broken state, this change fixes the same.

Bug: 160215626
Test: Incremental OTA now gets generated and applied
      successfully a non-ab device.

Change-Id: Ifb3886396fe65dcdaeba16c568419b9fa084193d
Merged-In: Ifb3886396fe65dcdaeba16c568419b9fa084193d