math.big: optimize subtract_digit_array()

[tankf33der]

Patch contains two parts:

1. added return for earlier exit if second input value is zero
2. attempt to remove 4 if statements plus two comparisons from the subtraction operation - the code became more linear and simple

Patch passed multi-million loop against gmplib.

[huly-for-github]

Connected to Huly®: V_0.6-23678