This code assumed `xd->version` was in the range `[0, 17]`. However, there is no
validation of this. The user can set an arbitrary version in the range
`[0, USHRT_MAX]` by using the `xdotversion` graph attribute. Setting a version
above 1.7 – or more specifically, that would be parsed as a value above 17 –
would cause an out of bounds read in this code.

Now any version above 1.7 is assumed to have no extensions; that is, equivalent
to version 1.4. There are no known planned extensions to xdot, so the user
should have no reason to set a version above 1.7.

Gitlab: fixes #2390