Defensive: Expect page that node writes onto to be zeroed.

I've seen data corruption that seen like a random bit-flip
or application on already allocated page.

Signed-off-by: Piotr Tabor <ptab@google.com>

node.go

@@ -188,7 +188,11 @@ func (n *node) read(p *page) {
 }
 
 // write writes the items onto one or more pages.
+// The page should have p.id (might be 0 for meta or bucket-inline page) and p.overflow set
+// and the rest should be zeroed.
 func (n *node) write(p *page) {
+	_assert(p.count == 0 && p.flags == 0, "node cannot be written into a not empty page")
+
 	// Initialize page.
 	if n.isLeaf {
 		p.flags = leafPageFlag