xz: Fix getopt_long argument type in --filters*

xz: Fix getopt_long argument type in --filters*

Forgetting the argument (or not using = to separate the option from
the argument) resulted in lzma_str_to_filters() being called with NULL
as input string argument. The function handles it fine but xz passes
the NULL to printf() too:

    $ xz --filters
    xz: Error in --filters=FILTERS option:
    xz: (null)
    xz: ^
    xz: Unexpected NULL pointer argument(s) to lzma_str_to_filters()

Now it's correct:

    $ xz --filters
    xz: option '--filters' requires an argument

The --filters-help option doesn't take any arguments.

Fixes: 9ded880a0221f4d1256845fc4ab957ffd377c760
Fixes: d6af7f347077b22403133239592e478931307759
Fixes: a165d7df1964121eb9df715e6f836a31c865beef
(cherry picked from commit 52ff32433734d03befd85a5bf00fba77d6501455)