format-date and dateTime index out of range

[mstevan]

@mstevan
test-set: format-date.xml, format-dateTime.xml
test-cases: format-date-013a to format-date-013n, format-dateTime-013a to format-dateTime-013v

@team
Seems that index is not working at all. Please investigate

Input:
let $t := date("0985-03-01") return format-date($t, "[Y,4-4]") -

Output:
[ERROR] An error has occurred: String index out of range: -1
We should investigate this 🙈. Please contact us or file an issue on GitHub with your query. 
Link: https://github.com/RumbleDB/rumble/issues

Expected output:
0985