Fix sign warnings for fill.
Fix sign warnings for fill.

Both precision and i are uint32_t, and memset()'s third parameter is
size_t, so we can safely change the type of fill to uint32_t.
The only other use is `index += fill;`, which doesn't trigger
sign warnings despite index being int.

ulfjack/ryu,ulfjack/ryu,ulfjack/ryu,ulfjack/ryu