Bluetooth: Controller: Fix PDU length in case of ISR latency issue

In case of ISR latencies, if packet pointer has not been
set on time then we do not want to check uninitialized
length in rx buffer that did not get used by Radio DMA.
This would help us in detecting radio ready event being
set? We can not detect radio ready if it happens twice
before Radio ISR executes after latency.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>