The problem is that in a MyISAM table the following column
after a varchar field gets corrupted, if varchar field is
extended.

This should be made to work without a copy in the future, but
I'm not sure if this code is ready yet. This fix will force copy
in this case. It will not do any harm to have it here, only makes
alter table a bit slower in this case. If this should work for
MyISAM, then the bug is somewhere else in that code.

Until it works, I propose this as a temporary fix or a workaround.
Test case for the bug has been added.