randr: clear primary screen's primaryOutput when the output is deleted

This fix use after free when a pluggable gpu screen (such as displaylink) was set as primary screen and unpluged.

gdb backtrace:

 #0 OssigHandler (signo=11, sip=0x7fff2e0a50f0, unused=0x7fff2e0a4fc0) at ../../../../os/osinit.c:138
 #1 <signal handler called>
 #2 rrGetscreenResources (client=0x3195160, query=0) at ../../../../randr/rrscreen.c:577
 #3 0x0000000000562bae in ProcRRGetscreenResourcesCurrent (client=0x3195160) at ../../../../randr/rrscreen.c:652
 #4 OxOOOOB0000054de63 in ProcRRDispatch (client=0x3195160) at ../../../../randr/randr.c:717
 #5 0x00000000004322c6 in Dispatch () at ../../../../dix/dispatch.c:485
 #6 0x0900900990443139 in dix_main (argc=12, argv=0x7fff2e0a5f78, envp=0x7fff2e0a5fe0) at ../../../../dix/main.c:276
 #7 0X0000000000421d9a in main (argc=12, argv=0x7fff2e0a5f78, envp=0x7fff2e0a5fe0) at ../../../../dix/stubmain.c:34

Signed-off-by: hongao hongao@uniontech.com