Ensure value is positive before casting size_t to uint64_t in calls to sxg_write_utf8_cbor_header

[twifkak]

Looks like all the callers of sxg_write_map_cbor_header pass in a size_t. For defense-in-depth, it would be nice to add an if (value < 0) do_something_else(); before each one is cast to a uint64_t.

Low risk. This isn't a buffer overflow risk. Worst case is we serve an invalid SXG that the browser rejects due to CBOR parse error.