Prevent get_hostname() from crashing.

get_hostname() crashes if "Match" is not followed by a
"Host" section in .ssh/config.

Closes paramiko#2339

paramiko/config.py

@@ -329,7 +329,8 @@ def get_hostnames(self):
         """
         hosts = set()
         for entry in self._config:
-            hosts.update(entry["host"])
+            if "host" in entry:
+                hosts.update(entry["host"])
         return hosts
 
     def _pattern_matches(self, patterns, target):

tests/configs/match-all

@@ -1,2 +1,3 @@
 Match all
     User awesome
+

tests/configs/match-include

@@ -0,0 +1,2 @@
+Match all
+Include ~/.ssh/config.extra

tests/test_config.py

@@ -717,6 +717,9 @@ def test_after_canonical_not_loaded_when_non_canonicalized(self, socket):
         result = load_config("match-canonical-no").lookup("a-host")
         assert "user" not in result
 
+    def test_match_include(self):
+        load_config("match-include")
+
 
 def _expect(success_on):
     """