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/signal_set.h b/runtime/signal_set.h
index 3b89e6e..c272514 100644
--- a/runtime/signal_set.h
+++ b/runtime/signal_set.h
@@ -38,7 +38,7 @@
   }
 
   void Block() {
-    if (sigprocmask(SIG_BLOCK, &set_, NULL) == -1) {
+    if (sigprocmask(SIG_BLOCK, &set_, nullptr) == -1) {
       PLOG(FATAL) << "sigprocmask failed";
     }
   }