Fix error handling

We log an error message, but then carry on and attempt to use the invalid impl, which then panics.

Change-Id: I440cdf728cb32479a10cbaecc5aadbdd97f215de
diff --git a/cc/ndk_library.go b/cc/ndk_library.go
index 49a919e..d704e32 100644
--- a/cc/ndk_library.go
+++ b/cc/ndk_library.go
@@ -307,6 +307,7 @@
 	impl, ok := dep.(*Module)
 	if !ok {
 		ctx.ModuleErrorf("Implementation for stub is not correct module type")
+		return nil
 	}
 	output := impl.UnstrippedOutputFile()
 	if output == nil {