Linecache contains support for PEP302 loaders, but fails to deal with loaders

that return None to indicate that the module is valid but no source is
available. This patch fixes that.

Lib/linecache.py

@@ -94,6 +94,10 @@ def updatecache(filename, module_globals=None):
                     except (ImportError, IOError):
                         pass
                     else:
+                        if data is None:
+                            # No luck, the PEP302 loader cannot find the source
+                            # for this module.
+                            return []
                         cache[filename] = (
                             len(data), None,
                             [line+'\n' for line in data.splitlines()], fullname