Explicitly encode path as UTF-8 in truncate_filename()

`os.pathconf()` expects a bytes object, but the path is stored as a
unicode object. Since there's no specified encoding, Python chokes when
trying to convert it to a bytes object. Explicitly encoding it as UTF-8
allows Python to change it to a bytes object smoothly.

Fixes https://github.com/whipper-team/whipper/issues/315

whipper/common/common.py

@@ -157,11 +157,11 @@ class MissingFrames(Exception):
 def truncate_filename(path):
     """
     Truncate filename to the max. len. allowed by the path's filesystem
-    Hopefully it handles Unicode strings correctly
     """
     p, f = os.path.split(os.path.normpath(path))
     f, e = os.path.splitext(f)
-    fn_lim = os.pathconf(p, 'PC_NAME_MAX')  # max.filename length in bytes
+    # Get the filename length limit in bytes
+    fn_lim = os.pathconf(p.encode('utf-8'), 'PC_NAME_MAX')
     f_max = fn_lim - len(e.encode('utf-8'))
     f = unicodedata.normalize('NFC', f)
     f_trunc = unicode(f.encode('utf-8')[:f_max], 'utf-8', errors='ignore')