Gracefully handle invalid tumblr responses

Querying for invalid usernames would result in a
reference error of body being undefined

Author: trueter

lib/request.js

@@ -48,5 +48,8 @@ req = (url, method = 'GET', fn, oauth) ->
     if not error and response.statusCode not in [200, 301]
       error = "#{response.statusCode} #{body.meta.msg}"
 
+    unless body.response?
+      error = "Invalid Response"
+
     if fn?
-      fn.call body, error, body.response
+      fn.call body, error, body?.response