MakeBigReq inserts a length field after the first 4 bytes of the request
 (after req->length), pushing everything else back by 4 bytes.

 The current memmove moves everything but the first 4 bytes back. If a
 request aligns to the end of the buffer pointer when MakeBigReq is
 invoked for that request, this runs over the buffer. Instead, we need to
 memmove minus the first 4 bytes (which aren't moved), minus the last 4
 bytes (so we still align to the previous tail).

 The 4 bytes that fell out are already handled with Data32, which will
 handle the buffermax correctly.

 The case where req->length = 1 was already not functional.

 Reported by Abhishek Arya <inferno@chromium.org> (against X.Org BTS).

 https://bugzilla.mozilla.org/show_bug.cgi?id=803762Reviewed-by: Jeff Muizelaar <jmuizelaar@mozilla.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
 Rebased-for-NX: Mike Gabriel <mike.gabriel@das-netzwerkteam.de>