Fix possible crash in nfs_block readdir

src/nfs/nfs_block.cpp

@@ -1056,11 +1056,15 @@ static void block_nfs3_readdir_common(void *opaque, rpc_op_t *rop, bool is_plus)
     uint64_t idx = 1;
     void *prev = NULL;
     for (auto it = entries.begin(); it != entries.end();)
+    {
+        if (args->cookie > 0 && idx <= args->cookie)
+            entries.erase(it++);
+        else
         {
             entryplus3 *entry = &it->second;
             // First fields of entry3 and entryplus3 are the same: fileid, name, cookie
             entry->name = xdr_copy_string(rop->xdrs, it->first);
-        entry->cookie = idx++;
+            entry->cookie = idx;
             if (prev)
             {
                 if (is_plus)
@@ -1069,11 +1073,10 @@ static void block_nfs3_readdir_common(void *opaque, rpc_op_t *rop, bool is_plus)
                     ((entry3*)prev)->nextentry = (entry3*)entry;
             }
             prev = entry;
-        if (args->cookie > 0 && entry->cookie == args->cookie)
-            entries.erase(entries.begin(), ++it);
-        else
             it++;
         }
+        idx++;
+    }
     // Now limit results based on maximum reply size
     // Sadly we have to calculate reply size by hand
     // reply without entries is 4+4+(dir_attributes ? sizeof(fattr3) : 0)+8+4 bytes