Missing error check for first PyIter_Next call in sendmsg_sendmsg

[exarkun]

On line 222, PyIter_Next is used to iterate over a list. When it returns NULL, we have to check to see if an error occurred, but we don't.

Note that the second call to PyIter_Next does do this checking (though the checking manifests as a rather mysterious PyErr_Occurred call, which is not obviously related to the PyIter_Next).

In practice, this is not a very likely case to come up. It should still be handled, though.