goprotobuf: Fix panic in SetDefaults for nil sub-messages.

Fixes #23.

R=r
CC=golang-dev
http://codereview.appspot.com/5477071
diff --git a/proto/all_test.go b/proto/all_test.go
index 1d64610..615e88a 100644
--- a/proto/all_test.go
+++ b/proto/all_test.go
@@ -1279,7 +1279,7 @@
 	}
 	SetDefaults(m)
 	if !Equal(m, expected) {
-		t.Errorf(" got %v\nwant %v", m, expected)
+		t.Errorf("\n got %v\nwant %v", m, expected)
 	}
 }