Add sanity check of query

contrib/tsearch2/rank.c

@@ -303,6 +303,7 @@ calc_rank_or(float *w, tsvector * t, QUERYTYPE * q)
 */
                 res = res + ( wjm + resj - wjm/((jm+1)*(jm+1)))/1.64493406685; 
 	}
+	if ( size > 0 )
 		res = res /size;
 	pfree(item);
 	return res;