segment: Improve segmentation when DEFINE command has early terminator.

The DEFINE command should not have a command terminator before the final
!ENDDEFINE.  When it does, that ends the whole thing early.  The segmenter
didn't handle this well, instead treating everything up to !ENDDEFINE as
part of the command.  That's probably not the best way because it makes
for confusing error messages.  This improves the behavior.