[#532] object/hash: Fix NPE during request forwarding

In current implementation `Object.GetRangeHash` RPC handler forwards range
requests for payload data. Missing request forwarder of the original request
caused NPE during execution.

Do not call request forwarder if payload range hash is requested.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/services/object/get/exec.go

@@ -38,7 +38,7 @@ type execCtx struct {
 
 	curOff uint64
 
-	head bool
+	head, hash bool
 
 	curProcEpoch uint64
 
@@ -71,6 +71,12 @@ func withPayloadRange(r *objectSDK.Range) execOption {
 	}
 }
 
+func hashOnly() execOption {
+	return func(c *execCtx) {
+		c.hash = true
+	}
+}
+
 func (exec *execCtx) setLogger(l *logger.Logger) {
 	req := "GET"
 	if exec.headOnly() {
@@ -146,6 +152,10 @@ func (exec *execCtx) headOnly() bool {
 	return exec.head
 }
 
+func (exec *execCtx) hashOnly() bool {
+	return exec.hash
+}
+
 func (exec *execCtx) netmapEpoch() uint64 {
 	return exec.prm.common.NetmapEpoch()
 }