codegen: Handle empty lines in overrides gracefully

Without this, having an empty line in an override will cause codegen to
unceremoniously choke to death.

https://bugzilla.gnome.org/show_bug.cgi?id=640341

codegen/override.py

@@ -101,6 +101,8 @@ class Overrides:
         else:
             line = buffer ; rest = ''
         words = string.split(line)
+        if len(words) == 0:
+            return
         command = words[0]
         if (command == 'ignore' or
             command == 'ignore-' + sys.platform):