avdevice: Use av_format_get_control_message_cb()

This is required as the location of this field could change and is
specified in libavformat not avdevice
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/avdevice.c

@@ -135,9 +135,9 @@ int avdevice_app_to_dev_control_message(struct AVFormatContext *s, enum AVAppToD
 int avdevice_dev_to_app_control_message(struct AVFormatContext *s, enum AVDevToAppMessageType type,
                                         void *data, size_t data_size)
 {
-    if (!s->control_message_cb)
+    if (!av_format_get_control_message_cb(s))
         return AVERROR(ENOSYS);
-    return s->control_message_cb(s, type, data, data_size);
+    return av_format_get_control_message_cb(s)(s, type, data, data_size);
 }
 
 int avdevice_capabilities_create(AVDeviceCapabilitiesQuery **caps, AVFormatContext *s,