Bug #31866: MySQL Server crashes on SHOW CREATE TRIGGER statement

SHOW CREATE TRIGGER was not checking for detected errors 
opening/reading the trigger file. 
Fixed to return the already generated error.

mysql-test/r/trigger.result

@@ -1978,3 +1978,9 @@ a
 1
 drop table table_25411_a;
 drop table table_25411_b;
+DROP TRIGGER IF EXISTS trg;
+Warnings:
+Note	1360	Trigger does not exist
+SHOW CREATE TRIGGER trg;
+ERROR HY000: Trigger does not exist
+End of 5.1 tests.

mysql-test/t/trigger.test

@@ -2246,3 +2246,15 @@ select * from table_25411_a;
 drop table table_25411_a;
 drop table table_25411_b;
 
+#
+# Bug #31866: MySQL Server crashes on SHOW CREATE TRIGGER statement
+#
+
+--disable-warnings
+DROP TRIGGER IF EXISTS trg;
+--enable-warnings
+
+--error ER_TRG_DOES_NOT_EXIST
+SHOW CREATE TRIGGER trg;
+
+--echo End of 5.1 tests.

sql/sql_show.cc

@@ -6862,6 +6862,9 @@ bool show_create_trigger(THD *thd, const sp_name *trg_name)
 {
   TABLE_LIST *lst= get_trigger_table(thd, trg_name);
 
+  if (!lst)
+    return TRUE;
+
   /*
     Open the table by name in order to load Table_triggers_list object.