Fix segfault when '[]' used for row_vector_property. * graphics.in.h (row_vector_property::add_constraint (octave_idx_type len)): Remove validation that accepts 0x0 matrix. * graphics.in.h (image::init): Call add_constraint() to allow 0x0 since xdata, ydata can legitimately be '[]' for these properties.

%!function r = f (x)
%!  if (ischar (x))
%!    r = x;
%!  else
%!    error ("expecting character string");
%!  endif
%!endfunction

%!assert (eval ("f 10;"), "10");