MDEV-30727 Check spider_hton_ptr in spider udfs

UDF isn't supposed to use my_error(), it should return
the error message in the provided error message buffer

Fixes valgrind:

==93993== Conditional jump or move depends on uninitialised value(s)
==93993==    at 0x484ECCD: strnlen (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==93993==    by 0x1AD2B2C: process_str_arg (my_vsnprintf.c:259)
==93993==    by 0x1AD47E0: my_vsnprintf_ex (my_vsnprintf.c:696)
==93993==    by 0x1A3B91E: my_error (my_error.c:120)
==93993==    by 0xF87BE8: udf_handler::fix_fields(THD*, Item_func_or_sum*, unsigned int, Item**) (item_func.cc:3638)

followup for 267dd5a9

storage/spider/mysql-test/spider/bugfix/r/mdev_30727.result

 CREATE FUNCTION spider_direct_sql RETURNS INT SONAME 'ha_spider.so';
 SELECT spider_direct_sql ('SELECT * FROM s','a','srv "b"');
-ERROR HY000: Plugin 'SPIDER' is not loaded
+ERROR HY000: Can't initialize function 'spider_direct_sql'; Plugin 'SPIDER' is not loaded
 CREATE FUNCTION spider_bg_direct_sql RETURNS INT SONAME 'ha_spider.so';
 SELECT spider_bg_direct_sql ('SELECT * FROM s','a','srv "b"');
-ERROR HY000: Plugin 'SPIDER' is not loaded
+ERROR HY000: Can't initialize function 'spider_bg_direct_sql'; Plugin 'SPIDER' is not loaded
 CREATE FUNCTION spider_copy_tables RETURNS INT SONAME 'ha_spider.so';
 SELECT spider_copy_tables ('t', '0', '0');
-ERROR HY000: Plugin 'SPIDER' is not loaded
+ERROR HY000: Can't initialize function 'spider_copy_tables'; Plugin 'SPIDER' is not loaded
 CREATE FUNCTION spider_flush_table_mon_cache RETURNS INT SONAME 'ha_spider.so';
 SELECT spider_flush_table_mon_cache ();
 spider_flush_table_mon_cache ()

storage/spider/mysql-test/spider/bugfix/t/mdev_30727.test

 CREATE FUNCTION spider_direct_sql RETURNS INT SONAME 'ha_spider.so';
---error ER_PLUGIN_IS_NOT_LOADED
+--error ER_CANT_INITIALIZE_UDF
 SELECT spider_direct_sql ('SELECT * FROM s','a','srv "b"');
 
 CREATE FUNCTION spider_bg_direct_sql RETURNS INT SONAME 'ha_spider.so';
---error ER_PLUGIN_IS_NOT_LOADED
+--error ER_CANT_INITIALIZE_UDF
 SELECT spider_bg_direct_sql ('SELECT * FROM s','a','srv "b"');
 
 CREATE FUNCTION spider_copy_tables RETURNS INT SONAME 'ha_spider.so';
---error ER_PLUGIN_IS_NOT_LOADED
+--error ER_CANT_INITIALIZE_UDF
 SELECT spider_copy_tables ('t', '0', '0');
 
 # spider_flush_table_mon_cache does not require spider init to function

storage/spider/spd_copy_tables.cc

@@ -1154,9 +1154,6 @@ long long spider_copy_tables_body(
   DBUG_RETURN(0);
 }
 
-#undef my_error
-extern "C" void my_error(unsigned int nr, unsigned long MyFlags, ...);
-
 my_bool spider_copy_tables_init_body(
   UDF_INIT *initid,
   UDF_ARGS *args,
@@ -1165,7 +1162,7 @@ my_bool spider_copy_tables_init_body(
   DBUG_ENTER("spider_copy_tables_init_body");
   if (!spider_hton_ptr)
   {
-    my_error(ER_PLUGIN_IS_NOT_LOADED, MYF(0), "SPIDER");
+    strcpy(message, "Plugin 'SPIDER' is not loaded");
     goto error;
   }
   if (args->arg_count != 3 && args->arg_count != 4)

storage/spider/spd_direct_sql.cc

@@ -1789,9 +1789,6 @@ long long spider_direct_sql_body(
   DBUG_RETURN(0);
 }
 
-#undef my_error
-extern "C" void my_error(unsigned int nr, unsigned long MyFlags, ...);
-
 my_bool spider_direct_sql_init_body(
   UDF_INIT *initid,
   UDF_ARGS *args,
@@ -1802,7 +1799,7 @@ my_bool spider_direct_sql_init_body(
   DBUG_ENTER("spider_direct_sql_init_body");
   if (!spider_hton_ptr)
   {
-    my_error(ER_PLUGIN_IS_NOT_LOADED, MYF(0), "SPIDER");
+    strcpy(message, "Plugin 'SPIDER' is not loaded");
     goto error;
   }
   if (args->arg_count != 3)