dix: Fix use after free in input device shutdown

This fixes access to freed heap memory via dev->master. E.g. when running BarrierNotify.ReceivesNotifyEvents/7 test from xorg-integration-tests:

==24736==ERROR: AddressSanitizer: heap-use-after-free on address
0x619000065020 at pc 0x55c450e2b9cf bp 0x7fffc532fd20 sp 0x7fffc532fd10
READ of size 4 at 0x619000065020 thread T0
    #0 0x55c450e2b9ce in GetMaster ../../../dix/devices.c:2722
    #1 0x55c450e9d035 in IsFloating ../../../dix/events.c:346
    #2 0x55c4513209c6 in GetDeviceUse ../../../Xi/xiquerydevice.c:525 ../../../Xi/xichangehierarchy.c:95
    #4 0x55c450e3455c in RemoveDevice ../../../dix/devices.c:1204 ../../../hw/xfree86/common/xf86Xinput.c:1142
    #6 0x55c450e17b04 in CloseDeviceList ../../../dix/devices.c:1038
    #7 0x55c450e1de85 in CloseDownDevices ../../../dix/devices.c:1068
    #8 0x55c450e837ef in dix_main ../../../dix/main.c:302
    #9 0x55c4517a8d93 in main ../../../dix/stubmain.c:34
(/lib/x86_64-linux-gnu/libc.so.6+0x28564)
    #11 0x55c450d0113d in _start (/usr/lib/xorg/Xorg+0x117713d)

0x619000065020 is located 160 bytes inside of 912-byte region
[0x619000064f80,0x619000065310)
freed by thread T0 here:
(/usr/lib/x86_64-linux-gnu/libasan.so.5+0x10d7cf)
    #1 0x55c450e19f1c in CloseDevice ../../../dix/devices.c:1014
    #2 0x55c450e343a4 in RemoveDevice ../../../dix/devices.c:1186 ../../../hw/xfree86/common/xf86Xinput.c:1142
    #4 0x55c450e17b04 in CloseDeviceList ../../../dix/devices.c:1038
    #5 0x55c450e1de85 in CloseDownDevices ../../../dix/devices.c:1068
    #6 0x55c450e837ef in dix_main ../../../dix/main.c:302
    #7 0x55c4517a8d93 in main ../../../dix/stubmain.c:34
(/lib/x86_64-linux-gnu/libc.so.6+0x28564)

previously allocated by thread T0 here:
(/usr/lib/x86_64-linux-gnu/libasan.so.5+0x10ddc6)
    #1 0x55c450e1c57b in AddInputDevice ../../../dix/devices.c:259
    #2 0x55c450e34840 in AllocDevicePair ../../../dix/devices.c:2755
    #3 0x55c45130318f in add_master ../../../Xi/xichangehierarchy.c:152 ../../../Xi/xichangehierarchy.c:465
    #5 0x55c4512cb9f5 in ProcIDispatch ../../../Xi/extinit.c:390
    #6 0x55c450e6a92b in Dispatch ../../../dix/dispatch.c:551
    #7 0x55c450e834b7 in dix_main ../../../dix/main.c:272
    #8 0x55c4517a8d93 in main ../../../dix/stubmain.c:34
(/lib/x86_64-linux-gnu/libc.so.6+0x28564)

The problem is caused by dev->master being not reset when disabling the device, which then causes dangling pointer when the master device itself is being deleted when exiting whole server.

Note that RecalculateMasterButtons() requires dev->master to be still valid, so we can reset it only at the end of function.