Error when querying database without all samples in RandomMC

[johnlees]

This occurs with repeated use of --update-db from PopPUNK, for example with blocks 1, 2, 3

• First DB contains sketches of 1, and RandomMC with reference keys from 1.
• Second DB contains sketchs of 1 + 2, but RandomMC is not updated and only has reference keys from 1.
• Third DB errors when computing a distance vs anything from 2, as its random match chance cannot be found