ec.c: Dispatch to `ecptxl_3' whether or not we have a curve.

ec.c: Dispatch to `ecptxl_3' whether or not we have a curve.

The `ecptxl_3' function has a better error message in the case where
there is no curve.  Indeed, `want sequence of two or three items' is
downright unfair in the case where you did in fact supply a sequence of
three items, and the real problem is that you're trying to construct a
curveless point.