BSSMAP_Emulation: ispresent(bssap.pdu.bssmap) before accessing fields

This seems not to be required on TITAN 6.3.0 on my laptop, but the
older 6.1.0 on Debian 9 seems to need it.

Change-Id: I574d8b79ac43e0fceddb3f9815666aef0ed66a3f
1 file changed