MDEV-12877 Wrong result from JSON native function.

        Set default charset for temporary paths so UDF call don't crash.

sql/item_jsonfunc.cc

@@ -650,6 +650,7 @@ static int alloc_tmp_paths(THD *thd, uint n_paths,
     if (*tmp_paths == 0)
     {
       MEM_ROOT *root= thd->stmt_arena->mem_root;
+
       *paths= (json_path_with_flags *) alloc_root(root,
           sizeof(json_path_with_flags) * n_paths);
       *tmp_paths= (String *) alloc_root(root, sizeof(String) * n_paths);
@@ -657,6 +658,8 @@ static int alloc_tmp_paths(THD *thd, uint n_paths,
         return 1;
 
       bzero(*tmp_paths, sizeof(String) * n_paths);
+      for (uint c_path=0; c_path < n_paths; c_path++)
+        (*tmp_paths)[c_path].set_charset(&my_charset_utf8_general_ci);
     }
 
     return 0;