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.

bolt_amd64.go

@@ -5,3 +5,7 @@ const maxMapSize = 0xFFFFFFFFFFFF // 256TB
 
 // maxAllocSize is the size used when creating array pointers.
 const maxAllocSize = 0x7FFFFFFF
+
+// Setting
+// const maxAllocSize = 70000
+// reveals the index out of bound bug(s)

node.go

@@ -220,12 +220,16 @@ func (n *node) write(p *page) {
 			elem.pgid = item.pgid
 			_assert(elem.pgid != p.id, "write: circular dependency occurred")
 		}
+		lk, lv := len(item.key), len(item.value)
+		if len(b) < lk+lv {
+			b = (*[maxAllocSize]byte)(unsafe.Pointer(&b[0]))[:]
+		}
 
 		// Write data for the element to the end of the page.
 		copy(b[0:], item.key)
-		b = b[len(item.key):]
+		b = b[lk:]
 		copy(b[0:], item.value)
-		b = b[len(item.value):]
+		b = b[lv:]
 	}
 
 	// DEBUG ONLY: n.dump()
@@ -351,7 +355,9 @@ func (n *node) spill() error {
 		}
 
 		// Allocate contiguous space for the node.
-		p, err := tx.allocate((node.size() / tx.db.pageSize) + 1)
+		// sz := node.size() + n.pageElementSize()*len(n.inodes)
+		sz := node.size()
+		p, err := tx.allocate((sz / tx.db.pageSize) + 1)
 		if err != nil {
 			return err
 		}

page.go

@@ -96,7 +96,7 @@ type branchPageElement struct {
 // key returns a byte slice of the node key.
 func (n *branchPageElement) key() []byte {
 	buf := (*[maxAllocSize]byte)(unsafe.Pointer(n))
-	return buf[n.pos : n.pos+n.ksize]
+	return (*[maxAllocSize]byte)(unsafe.Pointer(&buf[n.pos]))[:n.ksize]
 }
 
 // leafPageElement represents a node on a leaf page.
@@ -110,13 +110,13 @@ type leafPageElement struct {
 // key returns a byte slice of the node key.
 func (n *leafPageElement) key() []byte {
 	buf := (*[maxAllocSize]byte)(unsafe.Pointer(n))
-	return buf[n.pos : n.pos+n.ksize]
+	return (*[maxAllocSize]byte)(unsafe.Pointer(&buf[n.pos]))[:n.ksize]
 }
 
 // value returns a byte slice of the node value.
 func (n *leafPageElement) value() []byte {
 	buf := (*[maxAllocSize]byte)(unsafe.Pointer(n))
-	return buf[n.pos+n.ksize : n.pos+n.ksize+n.vsize]
+	return (*[maxAllocSize]byte)(unsafe.Pointer(&buf[n.pos+n.ksize]))[:n.vsize]
 }
 
 // PageInfo represents human readable information about a page.

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)
+		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]))
+		}
 	}
 	if !tx.db.NoSync || IgnoreNoSync {
 		if err := fdatasync(tx.db); err != nil {