Switch Coverity-only code to assert (CID #1619299) (#5441)

Switch Coverity-only code to assert (CID #1619299) (#5441)

fr_nbo_from_uint64v() does not have an error return--it doesn't
need one. The buffers are big enough, the "| 0x80" means it will
always use as least one byte, so fr_high_bit_pos() won't return 0
even if num == 0. So adding a bogus error return check for Coverity
actually misleads Coverity about any call to fr_nbo_from_uint64v(),
making it the probable cause of the CID.

Co-authored-by: Arran Cudbard-Bell <a.cudbardb@freeradius.org>