qdev: make release_tpm() idempotent

So it can eventually be called multiple times safely.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Link: https://lore.kernel.org/r/20250429140306.190384-4-marcandre.lureau@redhat.com
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20260108230311.584141-5-csomani@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

backends/tpm/tpm_util.c

@@ -72,6 +72,7 @@ static void release_tpm(Object *obj, const char *name, void *opaque)
 
     if (*be) {
         tpm_backend_reset(*be);
+        *be = NULL;
     }
 }