[#1664] write-cache: Fix panic on `Delete` operation

If an object is found in the Write-cache and is placed at the end of
the in-memory cache, the memory counter update operation tries to
dereference the index that is out of the sliced array. Moreover, even if
panic does not appear, the counter is updated with the wrong value.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

CHANGELOG.md

@@ -8,6 +8,7 @@ Changelog for NeoFS Node
 ### Changed
 
 ### Fixed
+- Panic on write-cache's `Delete` operation (#1664)
 
 ### Removed
 

pkg/local_object_storage/writecache/delete.go

@@ -26,9 +26,9 @@ func (c *cache) Delete(addr oid.Address) error {
 	c.mtx.Lock()
 	for i := range c.mem {
 		if saddr == c.mem[i].addr {
+			c.curMemSize -= uint64(len(c.mem[i].data))
 			copy(c.mem[i:], c.mem[i+1:])
 			c.mem = c.mem[:len(c.mem)-1]
-			c.curMemSize -= uint64(len(c.mem[i].data))
 			c.mtx.Unlock()
 			storagelog.Write(c.log, storagelog.AddressField(saddr), storagelog.OpField("in-mem DELETE"))
 			return nil