dix: Hold input lock for AttachDevice()

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/evdev_drv.so)
   by 0x485FDAC: EvdevReadInput (in /usr/lib64/xorg/modules/input/evdev_drv.so)
   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:2609.
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.

Closes: xorg/xserver#1260
Signed-off-by: Thomas Lindroth <thomas.lindroth@gmail.com>
4 jobs for fix-null-ptr-deref-in-mipointer.c in 5 minutes and 42 seconds (queued for 27 seconds)