[glib] gtlsbackend: Add missing preconditions for DTLS virtual methods

[Philip Withnall <pwithnall src gnome org>]

commit 52dd98475c45da92f971a15c6c9650b3bb1332ea
Author: Philip Withnall <withnall endlessm com>
Date:   Mon Sep 11 09:57:30 2017 +0100

    gtlsbackend: Add missing preconditions for DTLS virtual methods
    
    Make it a bit more obvious when the DTLS methods aren’t implemented by a
    particular TLS backend; return an invalid type rather than crashing.
    
    Signed-off-by: Philip Withnall <withnall endlessm com>
    
    https://bugzilla.gnome.org/show_bug.cgi?id=752240

 gio/gtlsbackend.c |   24 ++++++++++++++++++++----
 1 files changed, 20 insertions(+), 4 deletions(-)
---
diff --git a/gio/gtlsbackend.c b/gio/gtlsbackend.c
index a78d84b..db156c8 100644
--- a/gio/gtlsbackend.c
+++ b/gio/gtlsbackend.c
@@ -230,14 +230,22 @@ g_tls_backend_get_server_connection_type (GTlsBackend *backend)
  * Gets the #GType of @backend’s #GDtlsClientConnection implementation.
  *
  * Returns: the #GType of @backend’s #GDtlsClientConnection
- *   implementation.
+ *   implementation, or %G_TYPE_INVALID if this backend doesn’t support DTLS.
  *
  * Since: 2.48
  */
 GType
 g_tls_backend_get_dtls_client_connection_type (GTlsBackend *backend)
 {
-  return G_TLS_BACKEND_GET_INTERFACE (backend)->get_dtls_client_connection_type ();
+  GTlsBackendInterface *iface;
+
+  g_return_val_if_fail (G_IS_TLS_BACKEND (backend), G_TYPE_INVALID);
+
+  iface = G_TLS_BACKEND_GET_INTERFACE (backend);
+  if (iface->get_dtls_client_connection_type == NULL)
+    return G_TYPE_INVALID;
+
+  return iface->get_dtls_client_connection_type ();
 }
 
 /**
@@ -247,14 +255,22 @@ g_tls_backend_get_dtls_client_connection_type (GTlsBackend *backend)
  * Gets the #GType of @backend’s #GDtlsServerConnection implementation.
  *
  * Returns: the #GType of @backend’s #GDtlsServerConnection
- *   implementation.
+ *   implementation, or %G_TYPE_INVALID if this backend doesn’t support DTLS.
  *
  * Since: 2.48
  */
 GType
 g_tls_backend_get_dtls_server_connection_type (GTlsBackend *backend)
 {
-  return G_TLS_BACKEND_GET_INTERFACE (backend)->get_dtls_server_connection_type ();
+  GTlsBackendInterface *iface;
+
+  g_return_val_if_fail (G_IS_TLS_BACKEND (backend), G_TYPE_INVALID);
+
+  iface = G_TLS_BACKEND_GET_INTERFACE (backend);
+  if (iface->get_dtls_server_connection_type == NULL)
+    return G_TYPE_INVALID;
+
+  return iface->get_dtls_server_connection_type ();
 }
 
 /**