goprotobuf: Fixes for Clone and Equal.

R=r
CC=golang-dev
http://codereview.appspot.com/6419071
diff --git a/proto/text_parser.go b/proto/text_parser.go
index d4d5864..b06f403 100644
--- a/proto/text_parser.go
+++ b/proto/text_parser.go
@@ -416,8 +416,10 @@
 				return err
 			}
 
+			dst := sv.Field(fi)
+
 			// Parse into the field.
-			if err := p.readAny(sv.Field(fi), props); err != nil {
+			if err := p.readAny(dst, props); err != nil {
 				return err
 			}