xz: Flip the return value of suffix_is_set to match the documentation.

Also edit style to match the existing coding style in the project.

src/xz/args.c

@@ -726,9 +726,9 @@ args_parse(args_info *args, int argc, char **argv)
 
 	// If raw format is used and a custom suffix is not provided,
 	// then only stdout mode can be used when compressing or decompressing.
-	if (opt_format == FORMAT_RAW && suffix_is_set() && !opt_stdout &&
-			(opt_mode == MODE_COMPRESS ||
-			opt_mode == MODE_DECOMPRESS))
+	if (opt_format == FORMAT_RAW && !suffix_is_set() && !opt_stdout
+			&& (opt_mode == MODE_COMPRESS
+				|| opt_mode == MODE_DECOMPRESS))
 		message_fatal(_("With --format=raw, --suffix=.SUF is "
 				"required unless writing to stdout"));
 

src/xz/suffix.c

@@ -397,5 +397,5 @@ suffix_set(const char *suffix)
 extern bool
 suffix_is_set(void)
 {
-	return custom_suffix == NULL;
+	return custom_suffix != NULL;
 }

src/xz/suffix.h

@@ -27,6 +27,7 @@ extern char *suffix_get_dest_name(const char *src_name);
 /// and forgotten.
 extern void suffix_set(const char *suffix);
 
+
 /// \brief      Check if a custom suffix has been set
 ///
 /// Returns true if the internal tracking of the suffix string has been set