MDEV-25659 trigger name is empty after upgrade to 10.4

Problem:

At some point, we made stored rountines fail at CREATE time
instead of execution time in case of this syntax:

   IF unknown_variable
     ...
   END IF

As a result, a trigger created before this change and contained an unknown
variable worked in a bad way after upgrade:
- It was displayed with an empty trigger name by SHOW CREATE TRIGGER
- It was displayed with an empty trigger name by INFORMATION_SCHEMA.TRIGGERS
- An attempt to DROP this trigger returned errors - nothing happened.
- DROP TABLE did not remove the .TRN file corresponding to this broken trigger.

Underlying code observations:

The old code assumed that the trigger name resides in the current lex:

  if(thd->lex->spname)
    m_trigger_name= &thd->lex->spname->m_name;

This is not always the case. Some SP statements (e.g. IF)
do the following in their beginning:

- create a separate local LEX
- set thd->lex to this new local LEX
- push the new local LEX to the stack in sp_head::m_lex

and the following at the end of the statement:

- pop the previous LEX from the stack sp_head::m_lex
- set thd->lex back to the popped value

So when the parse error happens inside e.g. IF statement, thd->lex->spname
is a NULL pointer, because thd->lex points to the local LEX (without SP name)
rather than the top level LEX (with SP name).

Fix:
- Adding a new method sp_head::find_spname_recursive()
  which walks inside the LEX stack sp_head::m_lex from
  the top (the newest, most local) to the bottom (the oldest),
  and finds the one which contains a non-zero spname pointer.

- Using the new method inside
  Deprecated_trigger_syntax_handler::handle_condition():
  First it still tests thd->lex->spname (like before this change),
  and uses it in case it is not empty.
  Otherwise (if thd->lex->spname is empty), it calls
  sp_head::find_spname_recursive() to find the LEX with a
  non-empty spname inside the LEX stack of the current sphead.

sql/sql_trigger.cc

@@ -292,7 +292,7 @@ class Deprecated_trigger_syntax_handler : public Internal_error_handler
 private:
 
   char m_message[MYSQL_ERRMSG_SIZE];
-  LEX_CSTRING *m_trigger_name;
+  const LEX_CSTRING *m_trigger_name;
 
 public:
 
@@ -308,8 +308,23 @@ public:
     if (sql_errno != EE_OUTOFMEMORY &&
         sql_errno != ER_OUT_OF_RESOURCES)
     {
+      // Check if the current LEX contains a non-empty spname
       if(thd->lex->spname)
         m_trigger_name= &thd->lex->spname->m_name;
+      else if (thd->lex->sphead)
+      {
+        /*
+          Some SP statements, for example IF, create their own local LEX.
+          All LEX instances are available in the LEX stack in sphead::m_lex.
+          Let's find the one that contains a non-zero spname.
+          Note, although a parse error has happened, the LEX instances
+          in sphead::m_lex are not freed yet at this point. The first
+          found non-zero spname contains the valid trigger name.
+        */
+        const sp_name *spname= thd->lex->sphead->find_spname_recursive();
+        if (spname)
+          m_trigger_name= &spname->m_name;
+      }
       if (m_trigger_name)
         my_snprintf(m_message, sizeof(m_message),
                     ER_THD(thd, ER_ERROR_IN_TRIGGER_BODY),
@@ -322,7 +337,7 @@ public:
     return false;
   }
 
-  LEX_CSTRING *get_trigger_name() { return m_trigger_name; }
+  const LEX_CSTRING *get_trigger_name() { return m_trigger_name; }
   char *get_error_message() { return m_message; }
 };
 
@@ -1490,7 +1505,7 @@ bool Table_triggers_list::check_n_load(THD *thd, const LEX_CSTRING *db,
 
         if (unlikely(parse_error))
         {
-          LEX_CSTRING *name;
+          const LEX_CSTRING *name;
 
           /*
             In case of errors, disable all triggers for the table, but keep