Update math_eval.cpp

[Alcaro]

Still gives wrong answer for $4000000000000000*3 == $4000000000000000*3.0 (where former overflows to negative, but latter stays positive), but should fix db equal($5555555555555555, $5555555555555555+1) db equal($5555555555555555, $5555555555555555+1+511.0) to not return 00 01, at least