HPCC-12047 String repad causes seg fault on -ve lengths

Technically, this fix is incorrect, as someone COULD be trying to deal with
strings > 2Gb in length.

In practice, anyone trying to deal with strings that size will hit other
problems, and this can be fixed when those are.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
I Wasn't necessarily trying to find EVERY bug in stringlib...

Probably makes sense to fix this one now you've spotted it though