Properly use return value of select to count the number of events

src/iomux_select.c

@@ -168,17 +168,18 @@ again:
         }
 
         if (FD_ISSET(mux->descriptors[i]->fd, &read_set)) {
+            rc--;
             events |= MUX_EV_READ;
         }
 
         if (FD_ISSET(mux->descriptors[i]->fd, &write_set)) {
+            rc--;
             events |= MUX_EV_WRITE;
         }
         
         if (events) {
             assert(mux->descriptors[i]->callback != NULL);
             mux->descriptors[i]->callback(mux_, mux->descriptors[i], events);
-            rc--;
             event_cnt++;
         }
     }