internal/impl: better fast-path init checks for extensions

Unknown extensions are initialized.

Valid extensions with no isInit func are initialized.

Change-Id: I2975c7ef85d2b777eca467d3b1861d20de8e24fc
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/216960
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/internal/impl/decode.go b/internal/impl/decode.go
index 48f7ca5..290fc41 100644
--- a/internal/impl/decode.go
+++ b/internal/impl/decode.go
@@ -140,6 +140,9 @@
 			}
 			var o unmarshalOutput
 			o, err = mi.unmarshalExtension(b, num, wtyp, *exts, opts)
+			if err != nil {
+				break
+			}
 			n = o.n
 			if !o.initialized {
 				initialized = false
@@ -213,6 +216,9 @@
 	if err != nil {
 		return out, err
 	}
+	if xi.funcs.isInit == nil {
+		out.initialized = true
+	}
 	x.Set(xt, v)
 	exts[int32(num)] = x
 	return out, nil