[#322] node: Fix tree svc panic

If a connection has not been established earlier, it stores `nil` in LRU
cache. Cache eviction tries to close every connection (even a `nil` one) and
panics but not crash the app because we are using pools.
That ugly bug also leads to a deadlock where `Unlock` is not called via
`defer` func (and that is the way I found it).

Signed-off-by: Pavel Karpy <p.karpy@yadro.com>

CHANGELOG.md

@@ -12,6 +12,7 @@ Changelog for FrostFS Node
 - Take network settings into account during netmap contract update (#100)
 - Read config files from dir even if config file not provided via `--config` for node (#238)
 - Notary requests parsing according to `neo-go`'s updates (#268)
+- Tree service panic in its internal client cache (#322)
 
 ### Removed
 ### Updated

pkg/services/tree/cache.go

@@ -36,7 +36,9 @@ var errRecentlyFailed = errors.New("client has recently failed")
 
 func (c *clientCache) init() {
 	l, _ := simplelru.NewLRU[string, cacheItem](defaultClientCacheSize, func(_ string, value cacheItem) {
-		_ = value.cc.Close()
+		if conn := value.cc; conn != nil {
+			_ = conn.Close()
+		}
 	})
 	c.LRU = *l
 }