xz: Fix a bug with --files and --files0 in raw mode without a suffix.

The following command caused a segmentation fault:

    xz -Fraw --lzma1 --files=foo

when foo was a valid file. The usage of --files or --files0 was not
being checked when compressing or decompressing in raw mode without a
suffix. The suffix checking code was meant to validate that all files
to be processed are "-" (if not writing to standard out), meaning the
data is only coming from standard in. In this case, there were no file
names to check since --files and --files0 store their file name in a
different place.

Later code assumed the suffix was set and caused a segmentation fault.
Now, the above command results in an error.

src/xz/args.c

@@ -840,6 +840,11 @@ args_parse(args_info *args, int argc, char **argv)
 	if (opt_format == FORMAT_RAW && !suffix_is_set() && !opt_stdout
 			&& (opt_mode == MODE_COMPRESS
 				|| opt_mode == MODE_DECOMPRESS)) {
+		if (args->files_name != NULL)
+			message_fatal(_("With --format=raw, "
+					"--suffix=.SUF is required "
+					"unless writing to stdout"));
+
 		// If all of the filenames provided are "-" (more than one
 		// "-" could be specified) or no filenames are provided,
 		// then we are only going to be writing to standard out.