Fixes #94, with unspecified behavior in pointer comparisons.

[Alexhuszagh]

Fixes #94, by replacing unspecified pointer comparisons with well-defined behavior using std::distance with the current pointer and the pointer to one-past-the-end of the array.

As a simple schematic of why this is important, we have:

| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| x | x | x | p | - | - | - | - | - | - | - | - |

As you can if we have already the x digits, with an array with 10 elements, and try to compare p+8 to pend, we compare after the end of the array. This could optimize to being always true, or the integer arithmetic could wrap, leading to another always-true comparison.

The conformant solution is therefore: std::distance(p, pend) >= 8, which also optimizes well, so there's no issue of a performance hit (just ensuring the compiler does not cause an undesired optimizations due to unspecified behavior, such as a potential infinite loop).

[lemire]

Thanks for the detailed information. (I was convinced in about 3 seconds after reading the title of the issue, but it is always helpful to document the issue because other people read these things over time.)

[lemire]

Merged.