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/dex_instruction.h b/runtime/dex_instruction.h
index d3b9eb4..c64c21e 100644
--- a/runtime/dex_instruction.h
+++ b/runtime/dex_instruction.h
@@ -183,7 +183,7 @@
 
   // Reads an instruction out of the stream at the specified address.
   static const Instruction* At(const uint16_t* code) {
-    DCHECK(code != NULL);
+    DCHECK(code != nullptr);
     return reinterpret_cast<const Instruction*>(code);
   }