2013-06-24: Fix crash on nonexistent identity to pull from.

Fix crash on nonexistent identity to pull from.

diff --git a/infocalypse/commands.py b/infocalypse/commands.py
--- a/infocalypse/commands.py
+++ b/infocalypse/commands.py
@@ -196,6 +196,10 @@ def infocalypse_pull(ui_, repo, **opts):
         # an R0 if an R1 fails?
 
         repositories = wot.read_repo_listing(ui_, truster, wot_id)
+
+        if repositories is None:
+            return
+
         if repo_name not in repositories:
             ui_.warn("Could not find repository named \"{0}\".\n".format(repo_name))
             return