Add a function to unset the Bus' current sync handler

And use it in the Tokio example to unset the handler once the BusStream
is dropped.

examples/src/bin/tokio.rs

@@ -32,6 +32,12 @@ impl BusStream {
     }
 }
 
+impl Drop for BusStream {
+    fn drop(&mut self) {
+        self.0.unset_sync_handler();
+    }
+}
+
 impl Stream for BusStream {
     type Item = Message;
     type Error = ();

gstreamer/src/bus.rs

@@ -14,6 +14,7 @@ use glib::translate::*;
 use glib::source::{CallbackGuard, Continue, Priority, SourceId};
 use glib_ffi;
 use glib_ffi::{gboolean, gpointer};
+use std::ptr;
 
 use Bus;
 use BusSyncReply;
@@ -119,4 +120,15 @@ impl Bus {
             )
         }
     }
+
+    pub fn unset_sync_handler(&self) {
+        unsafe {
+            ffi::gst_bus_set_sync_handler(
+                self.to_glib_none().0,
+                None,
+                ptr::null_mut(),
+                None,
+            )
+        }
+    }
 }