goprotobuf: Detect and return a useful error if someone passes T (instead of *T) to proto.Marshal.

R=r
CC=golang-dev
http://codereview.appspot.com/4805047
diff --git a/proto/properties.go b/proto/properties.go
index 61d691e..44f4bc8 100644
--- a/proto/properties.go
+++ b/proto/properties.go
@@ -523,7 +523,7 @@
 	return prop.Prop[x[0]]
 }
 
-// Get the address and type of a pointer to the structure from an interface.
+// Get the address and type of a pointer to a struct from an interface.
 // unsafe.Reflect can do this, but does multiple mallocs.
 func getbase(pb interface{}) (t reflect.Type, b uintptr, err os.Error) {
 	// get pointer