Fix potential null pointer exceptions in msg.go

This commit adds conditions in msg.go file to check if the header 'HeaderDispositionNotificationTo' key exists in the map 'm.genHeader' before trying to associate it with a value. This prevents potential null pointer exceptions.

msg.go

@@ -520,7 +520,9 @@ func (m *Msg) RequestMDNTo(t ...string) error {
 		}
 		tl = append(tl, a.String())
 	}
-	m.genHeader[HeaderDispositionNotificationTo] = tl
+	if _, ok := m.genHeader[HeaderDispositionNotificationTo]; ok {
+		m.genHeader[HeaderDispositionNotificationTo] = tl
+	}
 	return nil
 }
 
@@ -541,7 +543,9 @@ func (m *Msg) RequestMDNAddTo(t string) error {
 	var tl []string
 	tl = append(tl, m.genHeader[HeaderDispositionNotificationTo]...)
 	tl = append(tl, a.String())
-	m.genHeader[HeaderDispositionNotificationTo] = tl
+	if _, ok := m.genHeader[HeaderDispositionNotificationTo]; ok {
+		m.genHeader[HeaderDispositionNotificationTo] = tl
+	}
 	return nil
 }