libostree: Fix potential use of uninitialised memory in progress API

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #835
Approved by: cgwalters

src/libostree/ostree-async-progress.c

@@ -331,6 +331,8 @@ ostree_async_progress_set (OstreeAsyncProgress *self,
   if (self->dead)
     goto out;
 
+  changed = FALSE;
+
   va_start (ap, self);
 
   for (key = va_arg (ap, const char *), format_string = va_arg (ap, const char *);