ui-snapshot.c: Fill argv[0] with dummy.

parse_archive_args does not pass PARSE_OPT_KEEP_ARGV0 to parse_args,
which means the first argument will be discarded, as though it were a
function being called from the command-line. Thus, we fill argv[0] with
a dummy argument to prevent this from happening.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>