Static vars in node_convert_field_convert don't clear between nodes

When you run a conversion on multiple nodes, say with VBO, the static variables $node_body and $node_teaser in node_convert_field_convert aren't reset for each node. This means that each node converted has it's content, plus the content of the nodes that were converted before it. I'm submitting a patch that checks the $nid argument against a static $_nid in node_convert_field_convert, and sets a $reset flag if they don't match. If the $reset flag is set, $node_body and $node_teaser are cleared and loaded with fresh data from the current node later in the function. This patch is against the current 6.x-1.x branch.