os: Don't crash in AttendClient if the client is gone

If a client is in the process of being closed down, then its client->osPrivate
pointer will be set to NULL by CloseDownConnection. This can cause a crash if
freeing the client's resources results in a call to AttendClient. For example,
if the client has a pending sync fence:

 Thread 1 "X" received signal SIGSEGV, Segmentation fault.
 AttendClient (client=0x5571c4aed9a0) at ../os/connection.c:942
 (gdb) bt
 #0  AttendClient (client=0x5571c4aed9a0) at ../os/connection.c:942
 #1  0x00005571c3dbb865 in SyncAwaitTriggerFired (pTrigger=<optimized out>) at ../Xext/sync.c:694
 #2  0x00005571c3dd5749 in miSyncDestroyFence (pFence=0x5571c5063980) at ../miext/sync/misync.c:120
 #3  0x00005571c3dbbc69 in FreeFence (obj=<optimized out>, id=<optimized out>) at ../Xext/sync.c:1909
 #4  0x00005571c3d7a01d in doFreeResource (res=0x5571c506e3d0, skip=skip@entry=0) at ../dix/resource.c:880
 #5  0x00005571c3d7b1dc in FreeClientResources (client=0x5571c4aed9a0) at ../dix/resource.c:1146
 #6  FreeClientResources (client=0x5571c4aed9a0) at ../dix/resource.c:1109
 #7  0x00005571c3d5525f in CloseDownClient (client=0x5571c4aed9a0) at ../dix/dispatch.c:3473
 #8  0x00005571c3d55eeb in Dispatch () at ../dix/dispatch.c:492
 #9  0x00005571c3d59e96 in dix_main (argc=3, argv=0x7ffe7854bc28, envp=<optimized out>) at ../dix/main.c:276
 #10 0x00007fea4837cb6b in __libc_start_main (main=0x5571c3d1d060 <main>, argc=3, argv=0x7ffe7854bc28, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffe7854bc18) at ../csu/libc-start.c:308
 #11 0x00005571c3d1d09a in _start () at ../Xext/sync.c:2378
 (gdb) print client->osPrivate
 $1 = (void *) 0x0

Since the client is about to be freed, its ignore count doesn't matter and
AttendClient can simply be a no-op. Check for client->clientGone in AttendClient
and remove similar checks from two callers that had them.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
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