ass_strtod: correctly convert large negative exponents

ass_strtod: correctly convert large negative exponents

Avoid overflow in dblExp that prevents subnormal numbers from being
generated (or small normal numbers if `double` supports many more
negative exponents than positive): if `10**abs(exp)` would overflow
and we actually want a negative exponent, switch to using precomputed
negative powers of 10 rather than positive.

Also avoid underflow for numbers with a large negative exponent where
the exponent alone underflows but the significand has enough digits to
cancel this out, e. g. in `10e-324` with IEEE 754 double.