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, 2 insertions, 2 deletions

diff --git a/netlib/utils.py b/netlib/utils.py
index 9eebf22c..0deb7c82 100644
--- a/netlib/utils.py
+++ b/netlib/utils.py
@@ -82,7 +82,7 @@ _label_valid = re.compile(b"(?!-)[A-Z\d-]{1,63}(?<!-)$", re.IGNORECASE)
 def is_valid_host(host):
     # type: (bytes) -> bool
     """
-    Checks if a hostname is valid.
+        Checks if a hostname is valid.
     """
     try:
         host.decode("idna")
@@ -90,7 +90,7 @@ def is_valid_host(host):
         return False
     if len(host) > 255:
         return False
-    if host[-1] == b".":
+    if host and host[-1:] == b".":
         host = host[:-1]
     return all(_label_valid.match(x) for x in host.split(b"."))