mobile/bind: handle null references from Java in Go

The Seq Java class has a special case for null references. Expand
the special case to Go so that null references from Java are properly
translated to nil.

Fixes golang/go#14228

Change-Id: I915d1f843c9db299d6910480f6d10dae0121a3b4
Reviewed-on: https://go-review.googlesource.com/19460
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/java/Seq.java

@@ -77,6 +77,8 @@ public class Seq {
 	public native void writeByteArray(byte[] v);
 
 	public void writeRef(Ref ref) {
+		if (ref == null)
+			ref = RefTracker.nullRef;
 		tracker.inc(ref);
 		writeInt32(ref.refnum);
 	}
@@ -172,9 +174,10 @@ public class Seq {
 
 	static final class RefTracker {
 		private static final int REF_OFFSET = 42;
+		private static final int NULL_REFNUM = 41; // also known to bind/seq/ref.go
 
 		// use single Ref for null Seq.Object
-		private static final Ref nullRef = new Ref(REF_OFFSET - 1, null);
+		private static final Ref nullRef = new Ref(NULL_REFNUM, null);
 
 		// Next Java object reference number.
 		//

bind/java/SeqTest.java

@@ -381,4 +381,12 @@ public class SeqTest extends AndroidTestCase {
     String got = n.getErr();
     assertEquals("want back the error message we set", want, got);
   }
+
+  public void testNullReferences() {
+    assertTrue(Testpkg.CallWithNull(null, new Testpkg.NullTest.Stub() {
+      public Testpkg.NullTest Null() {
+        return null;
+      }
+    }));
+  }
 }

bind/java/testpkg/testpkg.go

@@ -204,3 +204,11 @@ func ReadAsset() string {
 	}
 	return string(b)
 }
+
+type NullTest interface {
+	Null() NullTest
+}
+
+func CallWithNull(_null NullTest, nuller NullTest) bool {
+	return _null == nil && nuller.Null() == nil
+}