Extending a single-prec array with double-prec complex throws bad_cast.

[Jason Riedy]

And John W. Eaton writes:
> | octave -q --eval 'y = zeros (10, 1, "single"); x = ones (10, 1) + I (); y(:,2) = x;'

> I checked in the following change.
> http://hg.savannah.gnu.org/hgweb/octave/rev/ac8f5a159c2c

Works for me.  Thank you!

Jason