Make the memory layout of Port struct independent of USE_OPENSSL

Commit d39a49c1e4 added new fields to the struct, but missed the "keep
these last" comment on the previous fields. Add placeholder variables
so that the offsets of the fields are the same whether you build with
USE_OPENSSL or not. This is a courtesy to extensions that might peek
at the fields, to make the ABI the same regardless of the options used
to build PostgreSQL.

In reality, I don't expect any extensions to look at the 'raw_buf'
fields. Firstly, they are new in v17, so no one's written such
extensions yet. Secondly, extensions should have no business poking at
those fields anyway. Nevertheless, fix this properly on 'master'. On
v17, we mustn't change the memory layout, so just fix the comments.

Author: Jacob Champion
Discussion: https://www.postgresql.org/message-id/raw/CAOYmi%2BmKVJNzn5_TD_MK%3DhqO64r_w8Gb0FHCLk0oAkW-PJv8jQ@mail.gmail.com

src/include/libpq/libpq-be.h

@@ -207,12 +207,17 @@ typedef struct Port
 	bool		last_read_was_eof;
 
 	/*
-	 * OpenSSL structures. (Keep these last so that the locations of other
-	 * fields are the same whether or not you build with SSL enabled.)
+	 * OpenSSL structures.  As with GSSAPI above, to keep struct offsets
+	 * constant, NULL pointers are stored when SSL support is not enabled.
+	 * (Although extensions should have no business accessing the raw_buf
+	 * fields anyway.)
 	 */
 #ifdef USE_OPENSSL
 	SSL		   *ssl;
 	X509	   *peer;
+#else
+	void	   *ssl;
+	void	   *peer;
 #endif
 
 	/*