Fix bug in simplification of identically-zero chebtechs.

This addresses one of the points raised in #1283.  With the coefficients now ordered from lowest degree to highest, if we want to leave just one coefficient when our simplification yields an identically-zero result, we need to make that we set the last nonzero coefficient row to the _first_ row, not the last.  This modification should have been done a while ago but must have been missed when the coefficient ordering was flipped.

A test has been added to check for this problem in the future.