[distcc] distcc core dumps when called with "bad" cc options.

[Alexander Kolbasov]

When C compiler is called with options that are rejected by dcc_scan_args(), the
function returns without setting input_file. 

Later dcc_compile_local() is called with input_name set to NULL and core dumps
in dcc_critique_status() trying to log the file name and using %s on the NULL
pointer. 

Something like this fixes the problem:

static int dcc_compile_local(char *argv[],
                             char *input_name)
{
    pid_t pid;
    int ret;
    int status;

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

...

- akolb