Ignore nullptr in native_handle_close().

Just like native_handle_delete().

Bug: http://b/143898343
Test: new test
Change-Id: I373067aa6c2b03548b218c7011bdbf23bd71f260
diff --git a/libcutils/Android.bp b/libcutils/Android.bp
index 88e1bdb..334364e 100644
--- a/libcutils/Android.bp
+++ b/libcutils/Android.bp
@@ -197,7 +197,10 @@
 
 cc_defaults {
     name: "libcutils_test_default",
-    srcs: ["sockets_test.cpp"],
+    srcs: [
+        "native_handle_test.cpp",
+        "sockets_test.cpp",
+    ],
 
     target: {
         android: {