Fix loop condition in _allDistMargin

[twpayne]

I think this fixes a bug in _allDistMargin. In the existing code, compare the loop condition for leftBBox with the loop condition for rightBBox:

for (i = m; i < M - m; i++) { // leftBBox
    // ...
}

for (i = M - m - 1; i >= 0; i--) { // rightBBox
    // ...
}

The code for rightBBox completes more iterations. I think the code should be symmetrical with both loops completing the same number of iterations. I think the attached patch fixes this by making the rightBBox loop match the leftBBox loop.

[mourner]

You're totally right.