[SV 63609] Avoid buffer overrun in --warn-undefined-variables

[SV 63609] Avoid buffer overrun in --warn-undefined-variables

Reported by Dmitry Goncharov <dgoncharov@users.sf.net>

* src/variable.c (struct defined_vars): Create a struct that holds the
name and length of each variable name.
(warn_undefined): Check the lengths before comparing the contents.
* tests/scripts/options/warn-undefined-variables: Add a test.