do_summary_box() bugs?

[Ben Pfaff]

The "Clang" static analyzer found what looks to me like a bug in
do_summary_box(), in npar-summary.c.

It reports that "desc" can be a null pointer when it is
dereferenced in the "for" loop at the end.  Indeed, it looks to
me as if it can be, as long as "desc" can be null, and the code
itself thinks that it can be, since it tests whether it is
nonnull.

On my own, I think I see another problem: the code appears to add
headings for quartiles, but I do not see any code that would
insert the quartiles themselves.

Let me know what you think.  I'm happy to file a bug report if
that's what you want.
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