Avoid database lookup and update when new ram is not issued with every block.

[greg7mdp]

Add an early return if _gstate2.new_ram_per_block == 0 which is the case now:

See also in eos-system-contracts

reference-contracts/contracts/eosio.system/src/eosio.system.cpp

Lines 93 to 109 in 7718066

	void system_contract::update_ram_supply() {
	auto cbt = eosio::current_block_time();
	if( cbt <= _gstate2.last_ram_increase ) return;
	auto itr = _rammarket.find(ramcore_symbol.raw());
	auto new_ram = (cbt.slot - _gstate2.last_ram_increase.slot)*_gstate2.new_ram_per_block;
	_gstate.max_ram_size += new_ram;
	/**
	* Increase the amount of ram for sale based upon the change in max ram size.
	*/
	_rammarket.modify( itr, same_payer, [&]( auto& m ) {
	m.base.balance.amount += new_ram;
	});
	_gstate2.last_ram_increase = cbt;
	}