pp_repeat: handle large source strings for scalar repeat

pp_repeat: handle large source strings for scalar repeat

This deliberately dropped out since repeatcpy() took an I32 length
of source string parameter, but the code in repeatcpy() is
sufficiently robust we can simply make it a SSize_t.

Removed the error for strings over I32 len, the left over check
should reasonably handle integer overflow beyond the capacity of
a SSize_t.