Fix prepare_registration dust

We could get a rounding error in the contributor spots that would make
the registration fail when passed to oxend.  This fixes it by restoring
the missing logic that was accidentally dropped in the
prepare_registration refactor.

src/daemon/rpc_command_executor.cpp

@@ -2215,6 +2215,9 @@ bool rpc_command_executor::prepare_registration(bool force_registration)
           break;
         }
 
+        if (portions > state.portions_remaining)
+          portions = state.portions_remaining;
+
         state.contributions.push_back(portions);
         state.portions_remaining -= portions;
         state.total_reserved_contributions += get_actual_amount(staking_requirement, portions);