UBSan: Invalid static_cast from BinprotCommandT

UBSan complains about incorrect cast to derived in BinprotCommandT:

client_mcbp_commands.h:217:16: runtime error: downcast of address 0x7ffeefbfe5d0 which does not point to an object of type 'BinprotSaslAuthCommand'

0x7ffeefbfe5d0: note: object is of type 'BinprotCommandT<BinprotSaslAuthCommand, (unsigned char)33>'

I'm not totally sure this warning is accurate; the use of
static_cast<> here (CRTP) should be valid. However it (a) possibly
isn't, or (b) we are missing some RTTI information to allow UBSan to
check the cast.