Fix GetAddSubPrec calculation

[tompng]

x = BigDecimal('1' * 100) + 12345
# => 0.1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111123456e100
x.precs #=> [108, 234] → [108, 117]

Removes overflow/underflow checks. It won't happen.
a->Prec * SIZEOF_DECDIG is allocated memory size. fits in size_t. SIZEOF_DECDIG is 2 or 4.
a->exponent * BASE_FIG fits in ssize_t and BASE_FIG is 4 or 9.