robustness against double flush (merged from libsox)

src/rate.h

@@ -632,7 +632,7 @@ static void rate_flush(rate_t * p)
   size_t remaining = (size_t)(samples_out - p->samples_out);
   sample_t * buff = calloc(1024, sizeof(*buff));
 
-  if ((int)remaining > 0) {
+  if (samples_out > p->samples_out) {
     while ((size_t)fifo_occupancy(fifo) < remaining) {
       rate_input(p, buff, 1024);
       rate_process(p);