iso9660: fix 1-byte OOB read in iso-level option

iso9660_options() checks the second byte of the iso-level option value
before checking that the first byte contains a valid ISO level.

For an empty option value such as iso9660:iso-level=,
archive_write_set_options() passes a value pointer to the terminating NUL
byte of the duplicated options string. Reading value[1] then reads one byte
past that heap allocation.

Check value[0] before reading value[1]. This keeps the same accepted values
while rejecting the empty value without reading past the option buffer.

libarchive/archive_write_set_format_iso9660.c

@@ -1427,8 +1427,8 @@ iso9660_options(struct archive_write *a, const char *key, const char *value)
 		break;
 	case 'i':
 		if (strcmp(key, "iso-level") == 0) {
-			if (value != NULL && value[1] == '\0' &&
-			    (value[0] >= '1' && value[0] <= '4')) {
+			if (value != NULL && value[0] >= '1' &&
+			    value[0] <= '4' && value[1] == '\0') {
 				iso9660->opt.iso_level = value[0]-'0';
 				return (ARCHIVE_OK);
 			}