This patch is based on research done by Dmitry Osipenko to uncover the
cause of a large class of Xlib lockups.

_XError must unlock and re-lock the display around the call to the
user error handler function. When re-locking the display, two
functions are called to ensure that the display is ready to generate a request:

    _XIDHandler(dpy);
    _XSeqSyncFunction(dpy);

The first ensures that there is at least one XID available to use
(possibly calling _xcb_generate_id to do so). The second makes sure a
reply is received at least every 65535 requests to keep sequence
numbers in sync (possibly generating a GetInputFocus request and
synchronously awaiting the reply).

If the second of these does generate a GetInputFocus request and wait
for the reply, then a pending error will cause recursion into _XError,
which deadlocks the display.

One seemingly easy fix is to have _XError avoid those calls by
invoking InternalLockDisplay instead of LockDisplay....