fix `slice bounds out of range`/maxAllocSize bugs

when accessing the node data we used to use cast to
*[maxAllocSize]byte, which breaks if we try to go across maxAllocSize boundary.

This leads to occasional panics.
Sample stacktrace:
```
panic: runtime error: slice bounds out of range

goroutine 1 [running]:
github.com/boltdb/bolt.(*node).write(0xc208010f50, 0xc27452a000)
    $GOPATH/src/github.com/boltdb/bolt/node.go:228 +0x5a5
github.com/boltdb/bolt.(*node).spill(0xc208010f50, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/node.go:364 +0x506
github.com/boltdb/bolt.(*node).spill(0xc208010700, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/node.go:336 +0x12d
github.com/boltdb/bolt.(*node).spill(0xc208010620, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/node.go:336 +0x12d
github.com/boltdb/bolt.(*Bucket).spill(0xc22b6ae880, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/bucket.go:535 +0x1c4
github.com/boltdb/bolt.(*Bucket).spill(0xc22b6ae840, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/bucket.go:502 +0xac2
github.com/boltdb/bolt.(*Bucket).spill(0xc22f4e2018, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/bucket.go:502 +0xac2
github.com/boltdb/bolt.(*Tx).Commit(0xc22f4e2000, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/tx.go:150 +0x1ee
github.com/boltdb/bolt.(*DB).Update(0xc2080e4000, 0xc24d077508, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/db.go:483 +0x169
```
It usually happens when working with large (50M/100M) values.
One way to reproduce it is to change maxAllocSize in bolt_amd64.go to 70000 and run the tests.
TestBucket_Put_Large crashes.

1 files changed, 18 insertions, 6 deletions

diff --git a/tx.go b/tx.go
index fda6a21..9ffceaf 100644
--- a/tx.go
+++ b/tx.go
@@ -421,14 +421,26 @@ func (tx *Tx) write() error {
 	// Write pages to disk in order.
 	for _, p := range pages {
 		size := (int(p.overflow) + 1) * tx.db.pageSize
-		buf := (*[maxAllocSize]byte)(unsafe.Pointer(p))[:size]
 		offset := int64(p.id) * int64(tx.db.pageSize)
-		if _, err := tx.db.ops.writeAt(buf, offset); err != nil {
-			return err
+		ptr := (*[maxAllocSize]byte)(unsafe.Pointer(p))
+		for {
+			sz := size
+			if sz > maxAllocSize-1 {
+				sz = maxAllocSize - 1
+			}
+			buf := ptr[:sz]
+			if _, err := tx.db.ops.writeAt(buf, offset); err != nil {
+				return err
+			}
+			// Update statistics.
+			tx.stats.Write++
+			size -= sz
+			if size == 0 {
+				break
+			}
+			offset += int64(sz)
+			ptr = (*[maxAllocSize]byte)(unsafe.Pointer(&ptr[sz]))
 		}
-
-		// Update statistics.
-		tx.stats.Write++
 	}
 	if !tx.db.NoSync || IgnoreNoSync {
 		if err := fdatasync(tx.db); err != nil {