poll-common: Initialise g_autofree variables

These are both unconditionally assigned to before going out of scope, so
this is not a correctness fix as such – but initialising them now
prevents this potentially becoming a correctness issue in future, and
also stops future readers wondering if this is correct.

https://phabricator.endlessm.com/T35247

eos-updater/poll-common.c

@@ -386,7 +386,7 @@ checkpoint_blocked_event_payload_new (const char         *booted_ref,
                                       EuuCheckpointBlock  reason)
 {
   g_autoptr(GHashTable) hw_descriptors = NULL;
-  g_autofree gchar *vendor, *product;
+  g_autofree gchar *vendor = NULL, *product = NULL;
   const char *reason_nick;
 
   hw_descriptors = get_hw_descriptors ();