internal/errors: fix New in eliding prefix

Change-Id: I59c4c03f4115bfe6c4377924fadfd664245e878f
Reviewed-on: https://go-review.googlesource.com/c/164277
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/errors/errors.go b/internal/errors/errors.go
index 4fddf39..377d2ab 100644
--- a/internal/errors/errors.go
+++ b/internal/errors/errors.go
@@ -116,7 +116,7 @@
 // returns an error that has a "proto" prefix.
 func New(f string, x ...interface{}) error {
 	for i := 0; i < len(x); i++ {
-		if e, ok := x[i].(prefixError); ok {
+		if e, ok := x[i].(*prefixError); ok {
 			x[i] = e.s // avoid "proto: " prefix when chaining
 		}
 	}