Fix wrong pointer arithmetic when extending the file descripter list in the eventhandler

runtime/bin/eventhandler_macos.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include <errno.h>
 #include <poll.h>
 #include <pthread.h>
 #include <stdio.h>
 #include <string.h>
 #include <sys/time.h>
@@ skipping 53 matching lines @@
   if (fd >= port_map_size_) {
     intptr_t new_port_map_size = port_map_size_;
     do {
       new_port_map_size = new_port_map_size * kPortMapGrowingFactor;
     } while (fd >= new_port_map_size);
     size_t new_port_map_bytes = new_port_map_size * sizeof(PortData);
     port_map_ = reinterpret_cast<PortData*>(realloc(port_map_,
                                                     new_port_map_bytes));
     ASSERT(port_map_ != NULL);
     size_t port_map_bytes = port_map_size_ * sizeof(PortData);
-    memset(port_map_ + port_map_bytes,
+    memset(port_map_ + port_map_size_,
            0,
            new_port_map_bytes - port_map_bytes);
     port_map_size_ = new_port_map_size;
   }
 
   /*
    * Only change the port map entries count if SetPort changes
    * the port map state.
    */
   if (dart_port == 0 && PortFor(fd) != 0) {
@@ skipping 231 matching lines @@
     FATAL("Create start event handler thread");
   }
 }
 
 
 void EventHandlerImplementation::SendData(intptr_t id,
                                           Dart_Port dart_port,
                                           intptr_t data) {
   RegisterFdWakeup(id, dart_port, data);
 }