Avoid a warning.

Avoid a warning.

This fixes a warning introduced in #2449. There, @drwells had asked me
why I wrote
        for (std::size_t i=0; i!=N; ++i)
when I could have written
        for (std::size_t i=0; i<N; ++i)
I didn't remember and changed it to the latter, which did not warn on
my laptop but does now on the compute server. The reason is that N is
of type std::size_t, which turns out to be unsigned, and so warns if
we happen to have N==0. This patch fixes the issue.