Fix a bug in SpellCheckerService.

Previously, the returned value of WordIterator#following is used without
checking, which will cause fatal error when it's WordIterator.DONE.

Fix: 184915852
Test: atest CtsInputMethodTestCases:SpellCheckerTest
Change-Id: I37418bb36bc49e7dfa729ffb7209a6f383b611dd
1 file changed