Allow extra commas in filter-specific options on xz command line.

This may slightly ease writing scripts that construct
filter-specific option strings dynamically.

src/xz/options.c

@@ -69,6 +69,13 @@
 	char *name = s;
 
 	while (true) {
+		if (*name == ',') {
+			if (*++name == '\0')
+				break;
+
+			continue;
+		}
+
 		char *split = strchr(name, ',');
 		if (split != NULL)
 			*split = '\0';