shill: Replace NULL with nullptr.

BUG=None
TEST=`USE=wimax FEATURES=test emerge-$BOARD shill`

Change-Id: I30ab47ff32dcadad09ae7a2baf4d4123a6ef0d8e
Reviewed-on: https://chromium-review.googlesource.com/220657
Reviewed-by: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/dhcp_provider.cc b/dhcp_provider.cc
index 4106de0..0f6b4b3 100644
--- a/dhcp_provider.cc
+++ b/dhcp_provider.cc
@@ -27,10 +27,10 @@
 DHCPProvider::DHCPProvider()
     : proxy_factory_(ProxyFactory::GetInstance()),
       root_("/"),
-      control_interface_(NULL),
-      dispatcher_(NULL),
-      glib_(NULL),
-      metrics_(NULL) {
+      control_interface_(nullptr),
+      dispatcher_(nullptr),
+      glib_(nullptr),
+      metrics_(nullptr) {
   SLOG(DHCP, 2) << __func__;
 }
 
@@ -74,7 +74,7 @@
   SLOG(DHCP, 2) << __func__ << " pid: " << pid;
   PIDConfigMap::const_iterator it = configs_.find(pid);
   if (it == configs_.end()) {
-    return NULL;
+    return nullptr;
   }
   return it->second;
 }