udev: stop freeing value after using it for setting sysattr (#6094)

This prevents udev from double-freeing and crashing.

See https://github.com/systemd/systemd/issues/6040#issuecomment-306589836
==351== Invalid free() / delete / delete[] / realloc()
==351==    at 0x4C2C14B: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==351==    by 0x13CBE8: hashmap_clear_free_free (hashmap.c:900)
==351==    by 0x13CBE8: hashmap_free_free_free (hashmap.c:852)
==351==    by 0x147F4F: sd_device_unref (sd-device.c:88)
==351==    by 0x130CCC: udev_device_unref (libudev-device.c:552)
==351==    by 0x130CD5: udev_device_unref (libudev-device.c:553)
==351==    by 0x11FBBB: worker_spawn (udevd.c:488)
==351==    by 0x1216E5: event_run (udevd.c:584)
==351==    by 0x1216E5: event_queue_start (udevd.c:823)
==351==    by 0x122213: on_uevent (udevd.c:927)
==351==    by 0x141F2F: source_dispatch (sd-event.c:2272)
==351==    by 0x142D52: sd_event_dispatch (sd-event.c:2631)
==351==    by 0x142D52: sd_event_run (sd-event.c:2690)
==351==    by 0x142D52: sd_event_loop (sd-event.c:2710)
==351==    by 0x1159CB: run (udevd.c:1643)
==351==    by 0x1159CB: main (udevd.c:1772)
==351==  Address 0x81745b0 is 0 bytes inside a block of size 1 free'd
==351==    at 0x4C2C14B: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==351==    by 0x1447F0: freep (alloc-util.h:57)
==351==    by 0x1447F0: sd_device_set_sysattr_value (sd-device.c:1859)
==351==    by 0x132081: udev_device_set_sysattr_value (libudev-device.c:849)
==351==    by 0x12E777: set_trackpoint_sensitivity (udev-builtin-keyboard.c:180)
==351==    by 0x12E777: builtin_keyboard.lto_priv.170 (udev-builtin-keyboard.c:263)
==351==    by 0x14D03F: udev_builtin_run.constprop.75 (udev-builtin.c:133)
==351==    by 0x11FAEB: udev_event_execute_run (udev-event.c:957)
==351==    by 0x11FAEB: worker_spawn (udevd.c:461)
==351==    by 0x1216E5: event_run (udevd.c:584)
==351==    by 0x1216E5: event_queue_start (udevd.c:823)
==351==    by 0x122213: on_uevent (udevd.c:927)
==351==    by 0x141F2F: source_dispatch (sd-event.c:2272)
==351==    by 0x142D52: sd_event_dispatch (sd-event.c:2631)
==351==    by 0x142D52: sd_event_run (sd-event.c:2690)
==351==    by 0x142D52: sd_event_loop (sd-event.c:2710)
==351==    by 0x1159CB: run (udevd.c:1643)
==351==    by 0x1159CB: main (udevd.c:1772)
==351==  Block was alloc'd at
==351==    at 0x4C2CF35: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==351==    by 0x144853: sd_device_set_sysattr_value (sd-device.c:1888)
==351==    by 0x132081: udev_device_set_sysattr_value (libudev-device.c:849)
==351==    by 0x12E777: set_trackpoint_sensitivity (udev-builtin-keyboard.c:180)
==351==    by 0x12E777: builtin_keyboard.lto_priv.170 (udev-builtin-keyboard.c:263)
==351==    by 0x14D03F: udev_builtin_run.constprop.75 (udev-builtin.c:133)
==351==    by 0x11FAEB: udev_event_execute_run (udev-event.c:957)
==351==    by 0x11FAEB: worker_spawn (udevd.c:461)
==351==    by 0x1216E5: event_run (udevd.c:584)
==351==    by 0x1216E5: event_queue_start (udevd.c:823)
==351==    by 0x122213: on_uevent (udevd.c:927)
==351==    by 0x141F2F: source_dispatch (sd-event.c:2272)
==351==    by 0x142D52: sd_event_dispatch (sd-event.c:2631)
==351==    by 0x142D52: sd_event_run (sd-event.c:2690)
==351==    by 0x142D52: sd_event_loop (sd-event.c:2710)
==351==    by 0x1159CB: run (udevd.c:1643)
==351==    by 0x1159CB: main (udevd.c:1772)