[distcc] distcc core dumps when dcc_scan_args bails out

[Alexander Kolbasov]

When dcc_scan_args fails it doesn't set input_file. As a result,
dcc_compile_local() is called with input_name set to NULL and will core dump
later when dcc_critique_status() calls rs_log(). Adding something like this in
the beginning of dcc_compile_local helps:

    if (input_name == NULL)
        input_name = "(unknown)"; 

- akolb