That's for http://bugs.winehq.org/show_bug.cgi?id=20335 , right?
Alexandre's reply to that was "It sounds like it's hashing padding bytes. Probably should be reported to the X.org folks."
X is old code, and I don't think anyone has cleaned it up for valgrind yet.
That's why I use suppression files like http://winezeug.googlecode.com/svn/trunk/valgrind/valgrind-suppressions which contain blanket suppressions like { bogus_X11_1 Memcheck:Param write(buf) obj:* obj:/usr/lib*/libX11.so.6.2.0 } { bogus_X11_2 Memcheck:Param write(buf) obj:* obj:* obj:/usr/lib*/libX11.so.6.2.0 }