fix for the missed_blocks being too big causing UB

[entropycoset]

When trying to be a witness, and loading custom genesis where initial_timestamp is far behind, there seems to be an C++ UB
in dgp.recent_slots_filled = ... << missed_blocks
in https://github.com/bitshares/bitshares-core/blob/develop/libraries/chain/db_update.cpp#L68
because we left-shift by more than 126 (or 127, 128) bits there
https://timsong-cpp.github.io/cppwp/n4140/expr.shift#1 for C++14.

I suppose an explanation + abort is better than UB (or than trying to continue with settings the value to 0)

[abitmore]

recent_slot_filled is just used to calculate "witness participation rate", see https://github.com/bitshares/bitshares-core/blob/master/libraries/wallet/wallet_api_impl.cpp#L119 (IIRC it is also used in block production). Shifting and dropping the high bits is intended behavior. In my opinion UB on it is not serious at all and should not cause an abortion.

[abitmore]

Also see c019f18 for why it was implemented like that.

[entropycoset]

Then for the approach that sets resulting value to 0 (all bits are shifted away), and prints a warning - this PR: #2825