Bug 880: Prevent SIGSEGV in init_python when -no-home is used.

Before this patch, init_python would crash trying to set up elinks.home
at the Python side.  Now it uses None as the value in that case.
Also, init_python no longer adds "(null)" to $PYTHONPATH.

contrib/python/hooks.py

@@ -221,6 +221,9 @@ class feedreader:
 
     def __init__(self, feeds=my_favorite_feeds):
         """Constructor."""
+        if elinks.home is None:
+            raise elinks.error("Cannot identify unread entries without "
+                               "a ~/.elinks configuration directory.")
         self._results = {}
         self._feeds = feeds
         for feed in feeds: