[Spellcheck] Fix crash in no-suggestions case.
The FillSuggestions_OneLanguageNoSuggestions would crash with the old
FillSuggestions version, since it would loop essentially unbounded for
an empty suggestions vector.
The new version also simplifies the logic into a somewhat more clear
form. It runs potentially a few iterations more than the previous one,
but has easily understood bounds which will hopefully avoid further
under/overruns. (Famous last words! ;)
Also added a few more test cases for FillSuggestions, while I'm here.
BUG=524657
Review URL: https://codereview.chromium.org/
1308313007
Cr-Commit-Position: refs/heads/master@{#349171}