Remote: protect against invalid input on rename

The C API expects a non-NULL new name and raises an assertion if we
don't protect against such input, so let's guard against falsy values,
which also takes care of the empty string, which is also not valid
input.

pygit2/remote.py

@@ -144,6 +144,9 @@ class Remote(object):
 
     @name.setter
     def name(self, value):
+        if not value:
+            raise ValueError("New remote name must be a non-empty string")
+
         err = C.git_remote_rename(self._remote, to_str(value), ffi.NULL, ffi.NULL)
         check_error(err)
 

test/test_remote.py

@@ -65,6 +65,7 @@ class RepositoryTest(utils.RepoTestCase):
         self.assertEqual('new', remote.name)
 
         self.assertRaisesAssign(ValueError, remote, 'name', '')
+        self.assertRaisesAssign(ValueError, remote, 'name', None)
 
 
     def test_remote_set_url(self):