Fix unpacker doesn't raise exception for invalid input.

test/test_except.py

@@ -6,6 +6,7 @@ from msgpack import packb, unpackb
 
 import datetime
 
+
 class DummyException(Exception):
     pass
 
@@ -28,6 +29,11 @@ def test_raise_from_object_hook():
                   object_pairs_hook=hook)
 
 
+@raises(ValueError)
+def test_invalidvalue():
+    unpackb(b'\xd9\x97#DL_')
+
+
 if __name__ == '__main__':
     from nose import main
     main()