[#323] 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

@@ -7,6 +7,7 @@ Changelog for FrostFS Node
 ### Changed
 ### Fixed
 - Copy number was not used for `PUT` requests (#284)
+- Tree service panic in its internal client cache (#323)
 
 ### 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
 }