Check return value of tfw_hpack_exp_hdr

We should check that `tfw_hpack_exp_hdr` return not NULL
and return -ENOMEM if allocation fails to prevent null
dereference later.

Author: EvgeniiMekhanik

fw/hpack.c

@@ -1,7 +1,7 @@
 /**
  *		Tempesta FW
  *
- * Copyright (C) 2019-2024 Tempesta Technologies, Inc.
+ * Copyright (C) 2019-2025 Tempesta Technologies, Inc.
  *
  * This program is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License as published by
@@ -311,11 +311,16 @@ do {								\
 			r = -ENOMEM;				\
 			goto out;				\
 		}						\
-		if (!TFW_STR_EMPTY(&it->hdr))			\
+		if (!TFW_STR_EMPTY(&it->hdr)) {			\
 			it->next = tfw_hpack_exp_hdr(req->pool,	\
 						     length, it); \
-		else						\
+			if (!it->next) {			\
+				r = -ENOMEM;			\
+				goto out;			\
+			}					\
+		} else {					\
 			BUFFER_HDR_INIT(length, it);		\
+		}						\
 	}							\
 } while (0)