osx.m: avoid division by zero in startConfigureSheet.

When we set up a configuration sheet, we track the minimum overall
width that the controls will fit into (in a variable 'totalw'), and
separately, the minimum width needed by each of the left and right
columns containing control labels and actual controls ('leftw' and
'rightw'). If totalw > leftw+rightw at the end of the process, then we
must expand the two columns so that they have the right sum.

However, sometimes leftw+rightw can be zero, while totalw > 0. This
occurs if _no_ control in the box was of a type that used the left and
right columns for different things, so that the entire loop over the
controls only incremented totalw, and not leftw or rightw. For
example, in a puzzle such as Cube that defines no preferences of its
own, the only control in the preferences pane is midend.c's standard
"Keyboard shortcuts without Ctrl" preference, which is C_BOOLEAN and
only uses totalw.

In that situation, the code for proportionate distribution of the
excess divides by zero. So it needs a special case.

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