nimble/ll: Fix NULL pointer dereference on conn init cancel
nimble/ll: Fix NULL pointer dereference on conn init cancel

If connection was cancelled before isr_end we just set LL back to
standby and bail out.

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