fix(table_translator): index out of bound access in string

when trying to add blank to preedit then increase pos, might lead to
input[pos - 1] out of bound.

`pos` is tracking string variable `preedit`, they should be updated in
sync. checking delimiters at word ending should also refer to
`preedit` instead of the unchanged `input_`.

src/rime/gear/table_translator.cc

@@ -500,7 +500,7 @@ void SentenceTranslation::PrepareSentence() {
   // split syllables
   size_t pos = 0;
   for (int len : sentence_->word_lengths()) {
-    if (pos > 0 && delimiters.find(input_[pos - 1]) == string::npos) {
+    if (pos > 0 && delimiters.find(preedit[pos - 1]) == string::npos) {
       preedit.insert(pos, 1, ' ');
       ++pos;
     }