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/arch/x86/context_x86.h b/runtime/arch/x86/context_x86.h
index ace4670..a783d48 100644
--- a/runtime/arch/x86/context_x86.h
+++ b/runtime/arch/x86/context_x86.h
@@ -92,7 +92,7 @@
     XMM7_0, XMM7_1,
     kNumberOfFloatRegisters};
 
-  // Pointers to register locations. Values are initialized to NULL or the special registers below.
+  // Pointers to register locations. Values are initialized to null or the special registers below.
   uintptr_t* gprs_[kNumberOfCpuRegisters];
   uint32_t* fprs_[kNumberOfFloatRegisters];
   // Hold values for esp and eip if they are not located within a stack frame. EIP is somewhat