MDEV-21341: Fix UBSAN failures: Issue Six

(Variant #2 of the patch, which keeps the sp_head object inside the
MEM_ROOT that sp_head object owns)
(10.3 version of the fix, with handling for class sp_package)

sp_head::operator new() and operator delete() were dereferencing sp_head*
pointers to memory that didn't hold a valid sp_head object (it was
not created/already destroyed).
This caused UBSan to crash when looking up type information.

Fixed by providing static sp_head::create() and sp_head::destroy() methods.

sql/sql_trigger.cc

@@ -338,7 +338,7 @@ public:
 
 Trigger::~Trigger()
 {
-  delete body;
+  sp_head::destroy(body);
 }