xcb: Clarify the XInitThreads error message

XInitThreads has been called if _Xglobal_lock != NULL, we may as well
check that before printing a misleading error message.

Signed-off-by: Adam Jackson <ajax@redhat.com>
6 jobs for sequence-number-warning-fix in 9 minutes and 28 seconds (queued for 2 seconds)
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