verify rsa.Key object to prevent segfault

chef/api.py

@@ -75,6 +75,8 @@ def __init__(self, url, key, client, version='0.10.8', headers={}):
         self.parsed_url = urlparse.urlparse(self.url)
         if not isinstance(key, Key):
             key = Key(key)
+        if not key.key:
+            raise ValueError("ChefAPI attribute 'key' was invalid.")
         self.key = key
         self.client = client
         self.version = version

chef/tests/test_api.py

@@ -26,3 +26,9 @@ def test_env_variables(self):
             self.assertEqual(api.client, 'foobar')
         finally:
             del os.environ['_PYCHEF_TEST_']
+
+    def test_bad_key_raises(self):
+        invalids = [None, '']
+        for item in invalids:
+            self.assertRaises(
+                ValueError, ChefAPI, 'foobar', item, 'user')