Explicitly initialize ivar to nil to avoid warning
Explicitly initialize ivar to nil to avoid warning

Set `@index` to nil in Engine#initialize before encoding check.

Currently, if `check_haml_encoding` raises an exception, a warning is
issued in the rescue block as `@index` isn't yet initialized.

macressler/haml,nazar-pc/haml,tangposmarvin/haml,stanhu/haml,haml/haml,amatsuda/haml,tangposmarvin/haml,dg1dd1ngs/haml,dg1dd1ngs/haml,haml/haml,haml/haml,stanhu/haml,haml/haml,keithpitty/haml,macressler/haml,keithpitty/haml,sideci-sample/sideci-sample-haml,nazar-pc/haml