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

runtime/bin/eventhandler_linux.cc

Index: runtime/bin/eventhandler_linux.cc
diff --git a/runtime/bin/eventhandler_linux.cc b/runtime/bin/eventhandler_linux.cc
index 169ddbc6b3a41d8fa46b9d07e0c45790a657ff80..fded12e5cc818ea0855bae7743b6686dc2f86d50 100644
--- a/runtime/bin/eventhandler_linux.cc
+++ b/runtime/bin/eventhandler_linux.cc
@@ -71,7 +71,7 @@ void EventHandlerImplementation::SetPort(intptr_t fd,
                                                     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;