internal/impl: fix panic message in pointer.AsValueOf

Correctly report the type we were looking for when panicking.
Previously would say: "invalid type: got *T, want *T".

Change-Id: I90ea0dd1fc64f3aec37a5a5828c1832fb0ab2887
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/176258
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

internal/impl/pointer_reflect.go

@@ -51,8 +51,8 @@ func (p pointer) Apply(f offset) pointer {
 // AsValueOf treats p as a pointer to an object of type t and returns the value.
 // It is equivalent to reflect.ValueOf(p.AsIfaceOf(t))
 func (p pointer) AsValueOf(t reflect.Type) reflect.Value {
-	if p.v.Type().Elem() != t {
-		panic(fmt.Sprintf("invalid type: got %v, want %v", p.v.Type(), t))
+	if got := p.v.Type().Elem(); got != t {
+		panic(fmt.Sprintf("invalid type: got %v, want %v", got, t))
 	}
 	return p.v
 }