virstorageobj: Don't clear vols if they weren't initialized

If virStoragePoolObjNew() fails to create new volume object list
then virObjectUnref() is called and since refcounter is 1 then
virStoragePoolObjDispose() is called which in turn calls
virStoragePoolObjClearVols() which in turn dereferences
obj->volumes.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

src/conf/virstorageobj.c

@@ -619,6 +619,9 @@ virStoragePoolSourceFindDuplicateDevices(virStoragePoolObjPtr obj,
 void
 virStoragePoolObjClearVols(virStoragePoolObjPtr obj)
 {
+    if (!obj->volumes)
+        return;
+
     virHashRemoveAll(obj->volumes->objsKey);
     virHashRemoveAll(obj->volumes->objsName);
     virHashRemoveAll(obj->volumes->objsPath);