Switch from NULL -> nullptr.

Test: Builds.
Change-Id: Idcdd337e6576d612aa6c18a431b3dfe0215efde0
diff --git a/base/Status.cpp b/base/Status.cpp
index 3b31ee6..1ba91c3 100644
--- a/base/Status.cpp
+++ b/base/Status.cpp
@@ -53,7 +53,7 @@
     }
     std::string str = std::to_string(s);
     char *err = strerror(-s);
-    if (err != NULL) {
+    if (err != nullptr) {
         str.append(1, ' ').append(err);
     }
     return str;