master: fix possible crash during driver reload
master: fix possible crash during driver reload

This fixes a regression from commit ea4c2f39e79d4659c402b0fe9a9a1ea293aefd7c

The !is_loading condition was misplaced. It makes no sense to pass on a
heartbeat for a node that has no driver yet.

marel-keytech/openCANopen,marel-keytech/openCANopen,marel-keytech/openCANopen