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

[Jason Riedy]

The following terminates Octave's tip with a std::bad_cast from 
OPERATORS/op-fcm-fcm.cc:187:
  octave -q --eval 'y = zeros (10, 1, "single"); x = ones (10, 1) + I (); 
y(:,2) = x;'

Via gdb, the next level up where the values aren't optimized out
is octave_base_value::numeric_assign at ov-base.cc:1151. The rhs
appears to be of type "unknown", so the real error may have
occurred somewhere else.

Note that assigning to y(:,1) or using concatenation both work.

Jason