Fix netlib.utils.is_valid_host

- Don't crash when passed an empty string. This translated into an actual core
crash, discovered while fuzzing with afl.
- Taking a slice of length one out of bytes returns an integer, so the check
for trailing period in this function never worked on Python3.
- Add unit tests.

1 files changed, 7 insertions, 0 deletions

diff --git a/test/netlib/test_utils.py b/test/netlib/test_utils.py
index eaa66f13..f6acec03 100644
--- a/test/netlib/test_utils.py
+++ b/test/netlib/test_utils.py
@@ -3,6 +3,13 @@
 from netlib import utils, tutils
 
 
+def test_is_valid_host():
+    assert not utils.is_valid_host(b"")
+    assert utils.is_valid_host(b"one.two")
+    assert not utils.is_valid_host(b"one"*255)
+    assert utils.is_valid_host(b"one.two.")
+
+
 def test_bidi():
     b = utils.BiDi(a=1, b=2)
     assert b.a == 1