[Bug 225339] psm: Panic when trying to scroll with an Elantech touchpad without a trackpoint, with EVDEV_SUPPORT

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=225339

            Bug ID: 225339
           Summary: psm: Panic when trying to scroll with an Elantech
                    touchpad without a trackpoint, with EVDEV_SUPPORT
           Product: Base System
           Version: 11.0-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: dumbbell at FreeBSD.org

After fixing bug 225338 and enabling EVDEV_SUPPORT, I got a kernel panic when I
tried to scroll with the Elantech touch of a Gigabyte Aero 15X laptop:

    #4  0xffffffff80a947b3 in panic (fmt=<unavailable>) at
/mnt/home/dumbbell/Projects/freebsd/src/SVN/head/sys/kern/kern_shutdown.c:738
    #5  0xffffffff80f3ff32 in trap_fatal (frame=0xfffffe0087dc2610, eva=328) at
/mnt/home/dumbbell/Projects/freebsd/src/SVN/head/sys/amd64/amd64/trap.c:794
    #6  0xffffffff80f3ffa2 in trap_pfault (frame=0xfffffe0087dc2610,
usermode=0) at
/mnt/home/dumbbell/Projects/freebsd/src/SVN/head/sys/amd64/amd64/trap.c:649
    #7  0xffffffff80f3f772 in trap (frame=0xfffffe0087dc2610) at
/mnt/home/dumbbell/Projects/freebsd/src/SVN/head/sys/amd64/amd64/trap.c:417
    #8  <signal handler called>
    #9  0xffffffff8059e3d5 in bit_test (_bitstr=<optimized out>,
_bit=<optimized out>) at
/mnt/home/dumbbell/Projects/freebsd/src/SVN/head/sys/sys/bitstring.h:141
    #10 evdev_event_supported (type=2, evdev=<optimized out>) at
/mnt/home/dumbbell/Projects/freebsd/src/SVN/head/sys/dev/evdev/evdev.c:441
    #11 evdev_check_event (type=2, code=0, evdev=<optimized out>,
value=<optimized out>) at
/mnt/home/dumbbell/Projects/freebsd/src/SVN/head/sys/dev/evdev/evdev.c:490
    #12 evdev_push_event (evdev=0x0, type=2, code=0, value=-16) at
/mnt/home/dumbbell/Projects/freebsd/src/SVN/head/sys/dev/evdev/evdev.c:764
    #13 0xffffffff80f67546 in evdev_push_rel (evdev=<optimized out>,
code=<optimized out>, value=<optimized out>) at
/mnt/home/dumbbell/Projects/freebsd/src/SVN/head/sys/dev/evdev/evdev.h:172
    #14 proc_elantech (pb=<optimized out>, sc=<optimized out>, ms=<optimized
out>, x=<optimized out>, y=<optimized out>, z=<optimized out>)
        at
/mnt/home/dumbbell/Projects/freebsd/src/SVN/head/sys/dev/atkbdc/psm.c:4512

The problem is that the packet was erroneously detected as a trackpoint packet,
but the device has no trackpoint. We later want to emit an evdev event for this
packet, but the associated relative evdev device (`evdev_r`) is not
initialized, causing the panic.

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