glx: Fix potential crashes in glXWait{GL,X}

glxc->drawPriv will be NULL if the context is direct, or if it is
current but without a bound drawable. Mesa's libGL won't normally emit
protocol for direct contexts for these calls, but a malign client could
still crash the server.
3 jobs for glxwait-crash-fix in 6 minutes (queued for 17 seconds)