Fix sign warnings for len.
Fix sign warnings for len.

lengthForIndex() returns uint32_t (never an enormous value). The only
use of len is `for (int i = len - 1; i >= 0; --i)` which really wants
a signed value due to "countdown to 0" behavior. Therefore, we should
cast the return value of lengthForIndex() to int32_t. Additionally
for consistency, I'm changing the type of i from int to int32_t.

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