qtvcp: stop QPin update timer before hal_exit to prevent shutdown SIGSEGV

On Ubuntu 24.04 (glibc 2.39) shutting down a qtvcp screen can SIGSEGV
because the QPin polling timer keeps firing after hal_exit detaches HAL
shared memory.

Path: qtvcp.py shutdown() calls HAL.exit() -> _hal.component.exit() ->
hal_exit(comp_id). When the last component exits, hal_lib.c calls
rtapi_shmem_delete, unmapping HAL shmem. QPin.REGISTRY still holds Python
QPins whose pyhalitem.pin.u is a value-snapshot pointer into that shmem,
and the QPin.update_all QTimer is never stopped, so its next tick
dereferences an unmapped page.

Older glibc/kernel combinations happened to keep the pages around after
detach, hiding the bug.

Fix: stop the QPin update timer before HAL.exit().

Author: grandixximo

src/emc/usr_intf/qtvcp/qtvcp.py

@@ -523,6 +523,11 @@ def shutdown(self,signum=None,stack_frame=None):
         LOG.debug('Exiting HAL')
         if not HAL is None:
             try:
+                # Stop the QPin polling timer before hal_exit; otherwise its
+                # next tick dereferences pin->u after hal_exit has unmapped
+                # HAL shmem, causing SIGSEGV on glibc 2.39 (Ubuntu 24.04).
+                from qtvcp.qt_halobjects import QPin
+                QPin.update_stop()
                 HAL.exit()
             except Exception as e:
                 print(e)