mysqld got signal 11 in Item_field::fix_fields()

When I try to do an insert with a select, I am always able to cause mariadb server to exit on signal 11.

I do `docker run mariadb:10.3.* ` and then run the segfault.sql script as the root user. This causes the server to exit with signal 11.

This is a simplified example of an issue I have encountered in the wild.