BUG#18503734 - SEGV IN RBT_SEARCH(), FTS

1. Check whether cached_stopword is NULL in fts_tokenizer_word_get,
so that we can avoid segement fault. 2. Load stopwords if needed in
fts_add_doc_by_id. Note: The root cause why cached_stopword is not
initialized is still unfounded.

rb#6462 approved by Jimmy.

storage/innobase/fts/fts0fts.cc

@@ -1255,7 +1255,8 @@ fts_tokenizer_word_get(
 #endif
 
 	/* If it is a stopword, do not index it */
-	if (rbt_search(cache->stopword_info.cached_stopword,
+	if (cache->stopword_info.cached_stopword != NULL
+	    && rbt_search(cache->stopword_info.cached_stopword,
 		       &parent, text) == 0) {
 
 		return(NULL);
@@ -3557,6 +3558,12 @@ fts_add_doc_by_id(
 
 				rw_lock_x_lock(&table->fts->cache->lock);
 
+				if (table->fts->cache->stopword_info.status
+				    & STOPWORD_NOT_INIT) {
+					fts_load_stopword(table, NULL, NULL,
+							  NULL, TRUE, TRUE);
+				}
+
 				fts_cache_add_doc(
 					table->fts->cache,
 					get_doc->index_cache,