nvi bug fixing: memory use after free

[Sepherosa Ziehau]

Hi all,

nvi's "memory use after free" bug exists in following call path:
v_switch()->ex_edit()->set_alt_name()

Following patch fixes above bug:
http://leaf.dragonflybsd.org/~sephe/nvi.diff

The patch file is obtained from OpenBSD.

Not quite sure whether this is the correct way to patch our contrib source.
Please review it.

Best Regards,
sephe

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