`undef at end of file causes segfault in some cases.

[ben-marshall]

Issue Description

When parsing large numbers of files, a `undef directive appearing as the penultimate token in a file will cause a segfault. When running with valgrind however, it does not, so it is very much a heisenbug.

Steps to re-create:

Feed the parser a set of files, and one of them should have a `undef on the last line.

$> make clean
$> make all
$> ./src/parser ../../tests/*.v