Guard against PyNULL in arguments

[tkf]

fixes (the easier part of) #551

[stevengj]

Would be cleaner to have a function

function valid_pyobject(x)
    o = PyObject(x)
    ispynull(o) && error("invalid PyNULL object created from $x")
    return o
end

and call that in cases where we want to check for NULL results.

[stevengj]

An unexpected NULL object will generally indicate a bug somewhere else, as is the case here (#553), so I'm not sure it's worthwhile to insert these argument checks everywhere.

[tkf]

I saw ispynull(o) in pystring so I thought calling it makes sense. But I guess you have to use it there, as unless it's impossible to have PyNULL in REPL.

Anyway, I'm OK with not adding this support. Closing it.