Handle file-like objects with integer name attribute

When io.BufferedReader (and others) are init-ed based on a file
descriptor, their name attribute will be an integer. Handle these
objects by explicitly checking for an integer type in guess_format() and
setting sane values (empty strings).

We don't trap an exception because Python 2 and 3 throw different types;
AttributeError and TypeError, respectively. So, this way seemed cleaner.

libarchive/__init__.py

@@ -117,7 +117,10 @@ def get_func(name, items, index):
 
 
 def guess_format(filename):
-    filename, ext = os.path.splitext(filename)
+    if isinstance(filename, int):
+        filename = ext = ''
+    else:
+        filename, ext = os.path.splitext(filename)
     filter = FILTER_EXTENSIONS.get(ext)
     if filter:
         filename, ext = os.path.splitext(filename)