Check overflow in Loop_Integer

or this code will cause an infinite loop:

>> a: 9223372036854775807
== 9223372036854775807

>> for b 0 a to integer! a / 2 [print b]

With this commit:
>> for b 0 a to integer! a / 2 [print b]
0
4611686018427387904
** Math error: math or number overflow
** Where: for
** Near: for b 0 a to integer! a / 2 [print b]