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/lib.go b/proto/lib.go
index 33795e9..498cb75 100644
--- a/proto/lib.go
+++ b/proto/lib.go
@@ -657,7 +657,11 @@
 	}
 
 	for _, ni := range dm.nested {
-		setDefaults(v.Field(ni), recur, zeros)
+		f := v.Field(ni)
+		if f.IsNil() {
+			continue
+		}
+		setDefaults(f, recur, zeros)
 	}
 }