Ensure we deal with the case where N.domain gets assigned as a column vector

Previously, we returned not a particularly useful error message:

```
>> N = chebop(@(u) diff(u, 2) + u, [0; 2]);
>> N.bc = 0;
>> u = N\1
Error using bndfun (line 84)
Domain argument should be a row vector with two entries in increasing
order.
Error in classicfun.constructor (line 100)
                obj = bndfun(op, data, pref);
Error in fun.constructor (line 51)
                obj = classicfun.constructor(op, data, pref);
Error in chebfun/constructor>getFun (line 286)
g = fun.constructor(op, data, pref);
Error in chebfun/constructor>constructorNoSplit (line 110)
    [funs{k}, ishappy, data.vscale] = getFun(opk, endsk, data, pref);
Error in chebfun/constructor (line 63)
    [funs, ends] = constructorNoSplit(op, dom, data, pref);
Error in chebfun (line 221)
                [f.funs, f.domain] = chebfun.constructor(op, dom, data,
                pref);
Error in chebop/solvebvp (line 80)
    zeroFun = chebfun(0, dom);
Error in \ (line 13)
[varargout{1:nargout}] = solvebvp(varargin{:}); 
```