Mojo: Fix possibly-invalid vector subscripting in RawChannelPosix.

mojo/system/raw_channel_posix.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/system/raw_channel.h"
 
 #include <errno.h>
 #include <string.h>
 #include <unistd.h>
 
@@ skipping 278 matching lines @@
 
     // If we didn't max out |kReadSize|, stop reading for now.
     if (static_cast<size_t>(bytes_read) < kReadSize)
       break;
 
     // Else try to read some more....
   }
 
   // Move data back to start.
   if (read_buffer_start > 0) {
-    memmove(&read_buffer_[0], &read_buffer_[read_buffer_start],
-            read_buffer_num_valid_bytes_);
+    if (read_buffer_num_valid_bytes_ > 0) {
+      memmove(&read_buffer_[0], &read_buffer_[read_buffer_start],
+              read_buffer_num_valid_bytes_);
+    }
     read_buffer_start = 0;
   }
 }
 
 void RawChannelPosix::OnFileCanWriteWithoutBlocking(int fd) {
   DCHECK_EQ(fd, fd_);
   DCHECK_EQ(base::MessageLoop::current(), message_loop());
 
   bool did_fail = false;
   {
@@ skipping 90 matching lines @@
 // Static factory method declared in raw_channel.h.
 // static
 RawChannel* RawChannel::Create(const PlatformChannelHandle& handle,
                                Delegate* delegate,
                                base::MessageLoop* message_loop) {
   return new RawChannelPosix(handle, delegate, message_loop);
 }
 
 }  // namespace system
 }  // namespace mojo