clear rows before invoking nextRows callback

One more fix for the reader.

@joeferner. Sorry for bugging you one more time with this. I hope it will be the last one on this topic.

BTW, we are getting serious into using your driver in our application and I want to thank you for the great work you put into it. I like the design because it exposes a simple callback API without unnecessary bells and whistles. There are nevertherless a number of things that we'll need to add to it, like support for missing datatypes, streaming on BLOB and CLOB, etc. So you'll probably get more PR requests from us in the coming weeks.

Bruno