Fixes#1969.
The points-to analysis does not know that the assignment `input = raw_input`
cannot fail under Python 2, and so there are two possible values that `input`
could point-to after exiting the exception handler: the built-in `input`, or the
built-in `raw_input`. In the latter case we do not want to report the alert, and
so adding a check that the given function does not point-to the built-in
`raw_input` suffices.
