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/memory_region.cc b/runtime/memory_region.cc
index 06eba0f..a5c70c3 100644
--- a/runtime/memory_region.cc
+++ b/runtime/memory_region.cc
@@ -25,7 +25,7 @@
 namespace art {
 
 void MemoryRegion::CopyFrom(size_t offset, const MemoryRegion& from) const {
-  CHECK(from.pointer() != NULL);
+  CHECK(from.pointer() != nullptr);
   CHECK_GT(from.size(), 0U);
   CHECK_GE(this->size(), from.size());
   CHECK_LE(offset, this->size() - from.size());