Avoid range check failed in OnScanResult

Since sometime we received null adv data from nearby
devices, then the vector maybe empty. So we should use
member function data of vector to get the start address.
If we use operator [], then maybe receive a range checking
failure message, that will cause crash.

Test: No steps, just code refine
Change-Id: Ic578d41ae80ec6a254a42759341dcd115dfab12b