filter_nest: Replace malloc/free inside loop

Signed-off-by: Michiel Kalkman <mkalkman@shelde.com>

plugins/filter_nest/nest.c

@@ -278,20 +278,16 @@ static inline void pack_map(msgpack_packer * packer, msgpack_object * map,
                             struct filter_nest_ctx *ctx)
 {
     int i;
-    int size;
-    char *buf;
 
     msgpack_object *key;
 
     for (i = 0; i < map->via.map.size; i++) {
         key = &map->via.map.ptr[i].key;
 
         if (ctx->use_prefix) {
-            size = ctx->prefix_with_len + key->via.str.size;
-            buf = flb_malloc(size + 1);
-            snprintf(buf, size + 1, "%s%s", ctx->prefix_with, key->via.str.ptr);
-            helper_pack_string(packer, buf, size);
-            flb_free(buf);
+            msgpack_pack_str(packer, ctx->prefix_with_len + key->via.str.size);
+            msgpack_pack_str_body(packer, ctx->prefix_with, ctx->prefix_with_len);
+            msgpack_pack_str_body(packer, key->via.str.ptr, key->via.str.size);
         }
         else {
             msgpack_pack_object(packer, *key);