Correct bug description and ack commit

Add a reference to the relevant bug report, and ack the previously
unreviewed commit 9a06920db68d117f7ed68372e89a91536f60943f.

Acked-by: Guillem Jover <guillem@debian.org>

debian/changelog

@@ -3,7 +3,8 @@ dpkg (1.16.9) UNRELEASED; urgency=low
   [ Raphaël Hertzog ]
   * Fix dpkg-source regression in "3.0 (quilt)" source packages while
     unapplying patches that remove all files in a directory. Closes: #683547
-  * Fix segfault of “dpkg-query -W -f ''”.
+  * Fix segfault in field format parsing on empty strings, affecting
+    «dpkg-query -W -f ''» and «dpkg-deb -W --showformat=''». LP: #1035512
   * Fix dpkg's French usage string which was missing the final “s“ in
     --print-foreign-architectures. Closes: #685863