Fix a segfault that can occur in matchinfo if an fts4 table contains mostly zero-length documents. Specifically, if the table contains more rows than it does bytes of text.

FossilOrigin-Name: fe9047668eaaf76e7aa1ef1f32dec7c7c4226e45

ext/fts3/fts3_snippet.c

@@ -956,6 +956,7 @@ static int fts3MatchinfoSelectDoctotal(
     if( rc!=SQLITE_OK ) return rc;
   }
   pStmt = *ppStmt;
+  assert( sqlite3_data_count(pStmt)==1 );
 
   a = sqlite3_column_blob(pStmt, 0);
   a += sqlite3Fts3GetVarint(a, &nDoc);