Bad custom constructor behavior in #63 fix

[polesapart]

The fix for #63 as in pull-request #64 have problems.

The custom constructor takes some members from the target object but nullifies them afterwards:

database_binder(database_binder&& other) :
		_db(std::move(other._db)),
		_sql(std::move(other._sql)),
		_stmt(std::move(other._stmt)),
		_inx(other._inx), execution_started(other.execution_started) {
			_db = nullptr;
			_stmt = nullptr;
	}

In the initialization list it takes (moves) the db & stmt, but discards them on the body. The fix is to leave the body empty (I think the idea was to do other.{_db,_stmt} = nullptr instead, but that's not need as the move constructor from std::shared_ptr() will take care of that).

[aminroosta]

@polesapart You are absolutely right, my foolish mistake.
aminroosta@13b5c9e