[#510] treesvc: Fix panic in bearer token processing

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/services/tree/signature.go

@@ -110,7 +110,7 @@ func (s *Service) verifyClient(req message, cid cidSDK.ID, rawBearer []byte, op
 		}
 		tb = *tbCore.Value
 
-		if bt.Impersonate() {
+		if tableFromBearer && bt.Impersonate() {
 			signer = bt.SigningKeyBytes()
 		}
 	}