encoding/json: fix panic for nil instances of TextMarshaler in map keys

This change adds a a check in the encodeWithString.resolve method
to ensure that a reflect.Value with kind Ptr is not nil before
the type assertion to TextMarshaler.

If the value is nil, the method returns a nil error, and the map key
encodes to an empty string.

Fixes #33675

Change-Id: I0a04cf690ae67006f6a9c5f8cbb4cc99d236bca8
GitHub-Last-Rev: 6c987c9084
GitHub-Pull-Request: golang/go#33700
Reviewed-on: https://go-review.googlesource.com/c/go/+/190697
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

src/encoding/json/encode.go

@@ -932,6 +932,9 @@ func (w *reflectWithString) resolve() error {
 		return nil
 	}
 	if tm, ok := w.v.Interface().(encoding.TextMarshaler); ok {
+		if w.v.Kind() == reflect.Ptr && w.v.IsNil() {
+			return nil
+		}
 		buf, err := tm.MarshalText()
 		w.s = string(buf)
 		return err