MDEV-6799: Crash in field_conv, memcpy_field_possible

- Fix the crash, dont call from->type() at the start of the function
  because it might be unsafe.
- Unfortunately there is no testcase
- And this is also the reason we can't fix it properly (it should be
  safe to call from->type() here).

sql/field_conv.cc

@@ -837,7 +837,10 @@ bool memcpy_field_possible(Field *to,Field *from)
 {
   const enum_field_types to_real_type= to->real_type();
   const enum_field_types from_real_type= from->real_type();
-  const enum_field_types to_type= from->type();
+  /*
+    Warning: Calling from->type() may be unsafe in some (unclear) circumstances
+    related to SPs. See MDEV-6799.
+  */
   return (to_real_type == from_real_type &&
           !(to->flags & BLOB_FLAG && to->table->copy_blobs) &&
           to->pack_length() == from->pack_length() &&
@@ -850,8 +853,8 @@ bool memcpy_field_possible(Field *to,Field *from)
            to->field_length == from->field_length) &&
           from->charset() == to->charset() &&
           (!sql_mode_for_dates(to->table->in_use) ||
-           (to_type != MYSQL_TYPE_DATE &&
-            to_type != MYSQL_TYPE_DATETIME)) &&
+           (from->type()!= MYSQL_TYPE_DATE &&
+            from->type()!= MYSQL_TYPE_DATETIME)) &&
           (from_real_type != MYSQL_TYPE_VARCHAR ||
            ((Field_varstring*)from)->length_bytes ==
            ((Field_varstring*)to)->length_bytes));