Correctly count number of segments in _bus_dmamap_load().  Initialize ds_addr to 0UL instead of NULL while there.
Correctly count number of segments in _bus_dmamap_load().  Initialize ds_addr
to 0UL instead of NULL while there.

Tested by & help from dlg@.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars