guard zero digit count in GenerateCountedDigits

[Ramya-9353]

AddressSanitizer, -DNDEBUG build:

==ERROR: AddressSanitizer: heap-buffer-overflow ... WRITE of size 1
    #0 GenerateCountedDigits bignum-dtoa.cc:304
    #1 BignumDtoa bignum-dtoa.cc:161
located 1 bytes before 8-byte region [...]

Reached from the public API BignumDtoa(v, BIGNUM_DTOA_PRECISION, 0, buffer, &length, &point). GenerateCountedDigits only asserts count >= 0 and then stores the final digit at buffer[count - 1], so count == 0 writes buffer[-1]. DoubleToStringConverter::DoubleToAscii already special-cases precision mode with zero digits one layer up; BignumDtoa did not. Return an empty representation when count == 0, before the store.