MDEV-24524 Assertion `ls->length < 0xFFFFFFFFL && ((ls->length == 0 && !ls->str) || ls->length == strlen(ls->str))' failed in String::append on SELECT from I_S

don't expect return type of a stored function to be valid.
it's read from a table, so can be messed with.
it even can contain \0 bytes in the middle of the type name