stdio: remove erronous assert in dtoa

stdio: remove erronous assert in dtoa

The value of `k` in dtoa() is an estimate of
floor(log10(d)), where `d` is the number being
converted. The code was asserting that 'k' was
less than 100, but the full range of a double
goes to 10^308 or so.

This means that the majority of the range of
a double would trigger that assert.