Fix new -Wnull-conversion violations detected by a newer clang.

third_party/sudden_motion_sensor/sudden_motion_sensor_mac.cc

 // Copyright (c) 2012 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.
 //
 // This file is based on the SMSLib library.
 //
 // SMSLib Sudden Motion Sensor Access Library
 // Copyright (c) 2010 Suitable Systems
 // All rights reserved.
 //
@@ skipping 271 matching lines @@
 }
 
 // Probe the local hardware looking for a supported sensor device
 // and initialize an I/O connection to it.
 bool SuddenMotionSensor::Init() {
   // Request model name from the kernel.
   char local_model[32];  // size from SMSLib
   size_t local_model_size = sizeof(local_model);
   int params[2] = { CTL_HW, HW_MODEL };
   if (sysctl(params, 2, local_model, &local_model_size, NULL, 0) != 0)
-    return NULL;
+    return false;
 
   const SensorDescriptor* sensor_candidate = NULL;
 
   // Look for the current model in the supported sensor list.
   base::mac::ScopedCFTypeRef<CFDataRef> board_id_data;
   const int kNumSensors = arraysize(kSupportedSensors);
 
   for (int i = 0; i < kNumSensors; ++i) {
     // Check if the supported sensor model name is a prefix
     // of the local hardware model (empty names are accepted).
@@ skipping 88 matching lines @@
     if (value & 0x00800000)
       return value | 0xff000000;
     break;
 
   default:
     LOG(FATAL) << "Invalid integer size for sign extension: " << size;
   }
 
   return value;
 }