Add a check for invalid spans being set by SpellChecker.

One of the exit conditions of the loop immediately above the code is
when a BreakIterator finishes its input and emits BreakIterator.DONE,
which we were then mistakenly then using as a parameter for setting
spans.

Bug: 12936072
Bug: 5555929
Change-Id: Ib4aba113f2e7838cd040076160c1dd0fb90adb1a