Replace NULL with nullptr

Also fixed some lines that were too long, and a few other minor
details.

Change-Id: I6efba5fb6e03eb5d0a300fddb2a75bf8e2f175cb
diff --git a/runtime/gc/space/image_space.cc b/runtime/gc/space/image_space.cc
index e28e8d7..f350038 100644
--- a/runtime/gc/space/image_space.cc
+++ b/runtime/gc/space/image_space.cc
@@ -666,7 +666,7 @@
   }
 
   std::unique_ptr<File> file(OS::OpenFileForReading(image_filename));
-  if (file.get() == NULL) {
+  if (file.get() == nullptr) {
     *error_msg = StringPrintf("Failed to open '%s'", image_filename);
     return nullptr;
   }
@@ -695,7 +695,7 @@
   std::unique_ptr<MemMap> map(MemMap::MapFileAtAddress(
       image_header.GetImageBegin(), image_header.GetImageSize() + image_header.GetArtFieldsSize(),
       PROT_READ | PROT_WRITE, MAP_PRIVATE, file->Fd(), 0, false, image_filename, error_msg));
-  if (map.get() == NULL) {
+  if (map.get() == nullptr) {
     DCHECK(!error_msg->empty());
     return nullptr;
   }
@@ -786,7 +786,7 @@
                                     image_header.GetOatFileBegin(),
                                     !Runtime::Current()->IsAotCompiler(),
                                     nullptr, error_msg);
-  if (oat_file == NULL) {
+  if (oat_file == nullptr) {
     *error_msg = StringPrintf("Failed to open oat file '%s' referenced from image %s: %s",
                               oat_filename.c_str(), GetName(), error_msg->c_str());
     return nullptr;
@@ -811,7 +811,7 @@
 }
 
 bool ImageSpace::ValidateOatFile(std::string* error_msg) const {
-  CHECK(oat_file_.get() != NULL);
+  CHECK(oat_file_.get() != nullptr);
   for (const OatFile::OatDexFile* oat_dex_file : oat_file_->GetOatDexFiles()) {
     const std::string& dex_file_location = oat_dex_file->GetDexFileLocation();
     uint32_t dex_file_location_checksum;
@@ -837,7 +837,7 @@
 }
 
 OatFile* ImageSpace::ReleaseOatFile() {
-  CHECK(oat_file_.get() != NULL);
+  CHECK(oat_file_.get() != nullptr);
   return oat_file_.release();
 }