Patch: Escape character troubles (ITS#1753)

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Written by Hallvard B. Furuseth and placed into the public domain.
This software is not subject to any license of the University of Oslo.
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maildap could address buf[-1] if len was < 2.

REWRITE_SUBMATCH_ESCAPE is '%', not '\'.

librewrite and saslautz could walk past the end of a string which
ended with an escape character.

Hallvard B. Furuseth <h.b.furuseth@usit.uio.no>, April 2002.