stub: Fix NULL pointer deref when there are no initrds

stub: Fix NULL pointer deref when there are no initrds

When n_all_initrds == 0, then all_initrds is unmodified from its initial
value of:

	_cleanup_free_ struct iovec *all_initrds = NULL;

and in the else block of the "if (n_all_initrds > 1)" the NULL is
dereferenced:

		final_initrd = all_initrds[0];

Leading to the stub crashing due to a NULL pointer deref.

Fix this by initializing final_initrd to all 0s and only
running the else block if (n_all_initrds == 1).