Protect against key not found in genHeader map in msg.go

Added checks to ensure the key HeaderMessageID exists in the genHeader map before attempting to operate on it in msg.go. This will prevent possible null pointer exceptions when trying to assign the key a value or append it to the mids slice.

msg_test.go

@@ -758,7 +758,9 @@ func TestMsg_SetMessageIDWithValue(t *testing.T) {
 		t.Errorf("SetMessageID() failed. Expected value, got: empty")
 		return
 	}
+	if _, ok := m.genHeader[HeaderMessageID]; ok {
 		m.genHeader[HeaderMessageID] = nil
+	}
 	v := "This.is.a.message.id"
 	vf := "<This.is.a.message.id>"
 	m.SetMessageIDWithValue(v)
@@ -779,8 +781,10 @@ func TestMsg_SetMessageIDRandomness(t *testing.T) {
 		m := NewMsg()
 		m.SetMessageID()
 		mid := m.GetGenHeader(HeaderMessageID)
+		if len(mid) > 0 {
 			mids = append(mids, mid[0])
 		}
+	}
 	c := make(map[string]int)
 	for i := range mids {
 		c[mids[i]]++