Apparently it is possible to have chm file without #STRINGS in it, so we should not stop processing file at it. Fixes bug #12498

[Juan Lang]

>  Hmm, sorry, it is probably not a good idea to name a variable "null" -
>  it is just a little confusing. Maybe W_Null or even Wchar_Null or
>  to make it more obvious that it is a programmer-defined variable?

I agree, but Tomasz didn't name this variable, it was already in the
code--that's how I missed it.  I don't think it's a problem in this
patch.
--Juan