nimble/ll: Fix possible null pointer dereference
nimble/ll: Fix possible null pointer dereference

In case of empty beacon (no AUX_ADV_IND), there will be no aux_data.

This patch also:
- removes reduntant else as need_event is already reset at
the beggining of the loop

- fixes incorrect return parameter which could be set to 1 for
completed chained advertising event

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble