Bluetooth: Controller: Fix empty PDU buffer overrun under ISR latency

Only 3 bytes (PDU_EM_LL_SIZE_MAX) is required for empty PDU
transmission, but in case of Radio ISR latency if rx packet
pointer is not setup then Radio DMA will use previously
assigned buffer which can be this empty PDU buffer. Radio
DMA will overrun this buffer and cause memory corruption.
Any detection of ISR latency will not happen if the ISR
function pointer in RAM is corrupted by this overrun.
Increasing ISR latencies in OS and CPU usage in the
ULL_HIGH priority if it is same as LLL priority in
Controller implementation then it is making it tight to
execute Controller code in the tIFS between Tx-Rx PDU's
Radio ISRs.

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