ANDROID: Fix warning in libunpack jni.cpp

This is a partial backport of 8074839: Resolve disabled warnings for libunpack and the
unpack200 binary.

Change-Id: I2ca2f3d4102089a3ff30d2ecf53fb13a3f0b7eb9
diff --git a/src/share/native/com/sun/java/util/jar/pack/jni.cpp b/src/share/native/com/sun/java/util/jar/pack/jni.cpp
index e9109cb..f8ff76e 100644
--- a/src/share/native/com/sun/java/util/jar/pack/jni.cpp
+++ b/src/share/native/com/sun/java/util/jar/pack/jni.cpp
@@ -292,7 +292,7 @@
 
   if (uPtr->aborting()) {
     THROW_IOE(uPtr->get_abort_message());
-    return false;
+    return null;
   }
 
   // We have fetched all the files.