Prevent use after free in definemacro().

Prevent use after free in definemacro().
When the startup file contains (start-kbd-macro) mg crashes when
executing "start-kbd-macro" two times in the running mg.

The problem is that execline() is cleaning up behind our backs.
Finish the cleanup and make sure mg doesn't think we are in the
middle of a macro definition.

Problem found, tracked down and diff by gsoares@
Tweaked & OK by me.
Committing on request of gsoares@ who currently doesn't have good net
to commit.