gcancellable: Assert that make_pollfd() call succeeds

The `make_pollfd()` call can’t fail because it only does so if
`cancellable == NULL`, and we’ve already checked that. Assert that’s the
case, to shut Coverity up and to catch behavioural changes in future.

Coverity CID: #1159433
Signed-off-by: Philip Withnall <withnall@endlessm.com>

gio/gcancellable.c

@@ -348,6 +348,9 @@ int
 g_cancellable_get_fd (GCancellable *cancellable)
 {
   GPollFD pollfd;
+#ifndef G_OS_WIN32
+  gboolean retval;
+#endif
 
   if (cancellable == NULL)
 	  return -1;
@@ -355,7 +358,8 @@ g_cancellable_get_fd (GCancellable *cancellable)
 #ifdef G_OS_WIN32
   pollfd.fd = -1;
 #else
-  g_cancellable_make_pollfd (cancellable, &pollfd);
+  retval = g_cancellable_make_pollfd (cancellable, &pollfd);
+  g_assert (retval);
 #endif
 
   return pollfd.fd;