fix SIGSEGV in mipointer.c cause by null ptr deref race

Fix the following race:

Possible data race during read of size 8 at 0xA112510 by thread #6
Locks held: 1, at address 0x366B40
   at 0x14C8B9: GetMaster (devices.c:2691)
   by 0x15CFC5: IsFloating (events.c:346)
   by 0x2B9554: miPointerGetScreen (mipointer.c:527)
   by 0x1A5136: xf86PostButtonEventM (xf86Xinput.c:1379)
   by 0x1A52BD: xf86PostButtonEvent (xf86Xinput.c:1345)
   by 0x485F45B: EvdevProcessEvent (in /usr/lib64/xorg/modules/input/
   by 0x485FDAC: EvdevReadInput (in /usr/lib64/xorg/modules/input/
   by 0x195427: xf86ReadInput (xf86Events.c:247)
   by 0x2CC113: InputReady (inputthread.c:180)
   by 0x2CE4EA: ospoll_wait (ospoll.c:657)
   by 0x2CC077: InputThreadDoWork (inputthread.c:369)
   by 0x484A336: mythread_wrapper (hg_intercepts.c:406)

This conflicts with a previous write of size 8 by thread #1
Locks held: none
   at 0x14D2C6: AttachDevice (devices.c:2609)
   by 0x15CF85: ReattachToOldMaster (events.c:1457)
   by 0x1647DD: DeactivateKeyboardGrab (events.c:1700)
   by 0x25D7F1: ProcXIUngrabDevice (xigrabdev.c:169)
   by 0x2552AD: ProcIDispatch (extinit.c:398)
   by 0x155291: Dispatch (dispatch.c:479)
   by 0x158CBA: dix_main (main.c:276)
   by 0x143A3D: main (stubmain.c:34)
 Address 0xa112510 is 336 bytes inside a block of size 904 alloc'd
   at 0x4846571: calloc (vg_replace_malloc.c:1328)
   by 0x14A0B3: AddInputDevice (devices.c:260)
   by 0x1A31A0: xf86ActivateDevice (xf86Xinput.c:365)
   by 0x1A4549: xf86NewInputDevice (xf86Xinput.c:948)
   by 0x1A4B44: NewInputDeviceRequest (xf86Xinput.c:1090)
   by 0x1B81FE: device_added (udev.c:282)
   by 0x1B8516: config_udev_init (udev.c:439)
   by 0x1B7091: config_init (config.c:50)
   by 0x197970: InitInput (xf86Init.c:814)
   by 0x158C6B: dix_main (main.c:250)
   by 0x143A3D: main (stubmain.c:34)
 Block was alloc'd by thread #1

The steps to trigger the race are:

  1. Main thread does cleanup at mipointer.c:360 setting the slave device's miPointerPtr to null.
  2. Input thread use MIPOINTER in mipointer.c and get the slave's miPointerPtr = null.
  3. Main thread updates dev->master at devices.c:2632 (shown as line 2609 in trace above because of changes in master since the trace was captured).
  4. MIPOINTER would now return the master's miPointerPtr but the input thread already got the slave's miPointerPtr in step 2 and segfaults by null ptr deref.

Bug report with analysis: #1260 (closed)

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