Fix panic in ParseMessage

There is a chance the function can panic if a comma is included in the
input which is invalid.

envelopes_test.go

@@ -2,6 +2,7 @@ package nostr
 
 import (
 	"encoding/json"
+	"reflect"
 	"testing"
 )
 
@@ -110,3 +111,36 @@ func TestAuthEnvelopeEncodingAndDecoding(t *testing.T) {
 		}
 	}
 }
+
+func TestParseMessage(t *testing.T) {
+	testCases := []struct {
+		Name             string
+		Message          []byte
+		ExpectedEnvelope interface{}
+	}{
+		{
+			Name:             "nil",
+			Message:          nil,
+			ExpectedEnvelope: nil,
+		},
+		{
+			Name:             "invalid string",
+			Message:          []byte("invalid input"),
+			ExpectedEnvelope: nil,
+		},
+		{
+			Name:             "invalid string with a comma",
+			Message:          []byte("invalid, input"),
+			ExpectedEnvelope: nil,
+		},
+	}
+
+	for _, testCase := range testCases {
+		t.Run(testCase.Name, func(t *testing.T) {
+			envelope := ParseMessage(testCase.Message)
+			if !reflect.DeepEqual(testCase.ExpectedEnvelope, envelope) {
+				t.Fatal("unexpected output")
+			}
+		})
+	}
+}