clamp RadixStringToIeee exponent accumulation against signed overflow

[jdymitarai]

Problem

Similar to #310 (clamp StringToIeee exponent accumulation against signed overflow), RadixStringToIeee in double-conversion/string-to-double.cc accumulates the exponent across multiple stages without sufficient bounds:

1. Fractional digits in hex-floats decrement exponent -= radix_log_2 without a lower floor, allowing pathologically long fractional inputs to underflow INT_MIN (signed integer overflow/underflow, which is undefined behavior).
2. Dropped integer digits past the significand limit previously accumulated with a cap at INT_MAX. When subsequently adding the parsed exponent field (exponent += written_exponent), any positive written_exponent triggers signed integer overflow (INT_MAX + written_exponent).
3. Calling ldexp(static_cast<double>(number), exponent) with extreme exponents can trip sanitizers or platform-dependent math behavior.

Solution

1. Floor the post-decimal fractional exponent decrement at -(max_exponent / 2) where max_exponent = INT_MAX / 2, mirroring the floor in StringToIeee (clamp StringToIeee exponent accumulation against signed overflow #310).
2. Clamp dropped integer digits accumulation to max_exponent instead of INT_MAX.
3. Combine exponent and written_exponent in int64_t and clamp to [-max_exponent, max_exponent].
4. Guard ldexp calls by returning +/-Infinity when exponent > 100 * Double::kMaxExponent and SignedZero(sign) when exponent < -100 * Double::kMaxExponent.
5. Add regression test cases for extreme exponents in StrToD and StrToF to test/cctest/test-conversions.cc.

[floitsch]

LGTM.