BUG #15956: Server closed unexpectedly for user-defined base type LIKE char

The following bug has been logged on the website:

Bug reference:      15956
Logged by:          Jason Kim
Email address:      jj-kim@users.noreply.github.com
PostgreSQL version: 11.4
Operating system:   OS X (and CentOS)
Description:

The following causes the server to close unexpectedly.

```
CREATE TYPE char_type;
CREATE FUNCTION char_type_in(cstring) RETURNS char_type
   LANGUAGE internal IMMUTABLE STRICT AS 'charin';
CREATE FUNCTION char_type_out(char_type) RETURNS cstring
   LANGUAGE internal IMMUTABLE STRICT AS 'charout';
CREATE TYPE char_type (
   INPUT = char_type_in,
   OUTPUT = char_type_out,
   LIKE = char
);
CREATE TABLE char_table (t char_type);
INSERT INTO char_table (t)
  VALUES ('t');
```

Here is the server close message:

```
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!> \q
```

This is tested using Postgres v11.4 on OS X and Postgres v9.2.24 on CentOS.