fix issue #94 ：need to close db connection if res is nil.

[zhiyong0804]

add db:close when query failed.

res, err, errcode, sqlstate = db:query("select 1; select 2; select 3;") if not res then ngx.log(ngx.ERR, "bad result #1: ", err, ": ", errcode, ": ", sqlstate, ".") db:close() return ngx.exit(500) end

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This change is