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>
Other plugins do. Stringlib used to be purer than most, but since we decided that plugins were versioned with the system I think it's ok.