client.py: avoid an exception when no new remote packs were generated.

This is probably pretty rare, but it can happen if you needed to download a
remote index, and that index had *all* your objects, so we did end up
writing some objects to the remote server, but it didn't end up generating
any packs.  If that happened, we would try to return the contents of a
nonexistent variable.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>

lib/bup/client.py

@@ -222,6 +222,7 @@ def _suggest_packs(self):
         self.check_ok()
         if ob:
             self._busy = None
+        idx = None
         for idx in suggested:
             self.sync_index(idx)
         git.auto_midx(self.cachedir)