fmt: adjust formatting of invalid reflect.Value, add more tests

Repeat of CL 8951.

Change-Id: I5430e4a9eb5d8b7d0e3963657092bede67439056
Reviewed-on: https://go-review.googlesource.com/9003Reviewed-by: Rob Pike <r@golang.org>

src/fmt/fmt_test.go

@@ -686,6 +686,14 @@ var fmtTests = []struct {
 	// Issue 8965.
 	{"%v", reflect.ValueOf(A{}).Field(0).String(), "<int Value>"}, // Equivalent to the old way.
 	{"%v", reflect.ValueOf(A{}).Field(0), "0"},                    // Sees inside the field.
+
+	// verbs apply to the extracted value too.
+	{"%s", reflect.ValueOf("hello"), "hello"},
+	{"%q", reflect.ValueOf("hello"), `"hello"`},
+	{"%#04x", reflect.ValueOf(256), "0x0100"},
+
+	// invalid reflect.Value doesn't crash.
+	{"%v", reflect.Value{}, "<invalid reflect.Value>"},
 }
 
 // zeroFill generates zero-filled strings of the specified width. The length

src/fmt/print.go

@@ -847,6 +847,8 @@ func (p *pp) printReflectValue(value reflect.Value, verb rune, depth int) (wasSt
 	p.value = value
 BigSwitch:
 	switch f := value; f.Kind() {
+	case reflect.Invalid:
+		p.buf.WriteString("<invalid reflect.Value>")
 	case reflect.Bool:
 		p.fmtBool(f.Bool(), verb)
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64: