pg_upgrade: Fix inconsistency in memory freeing

The function in charge of freeing the memory from a result created by
PQescapeIdentifier() has to be PQfreemem(), to ensure that both
allocation and free come from libpq.

One spot in pg_upgrade was not respecting that for pg_database's
datlocale (daticulocale in v16) when the collation provider is libc (aka
datlocale/daticulocale is NULL) with an allocation done using
pg_strdup() and a free with PQfreemem().  The code is changed to always
use PQescapeLiteral() when processing the input.

Oversight in 9637bad.  This commit is similar to 48e4ae9 and
5b94e27.

Author: Michael Paquier <michael@paquier.xyz>
Co-authored-by: Ranier Vilela <ranier.vf@gmail.com>
Discussion: https://postgr.es/m/Z601RQxTmIUohdkV@paquier.xyz
Backpatch-through: 16

Author: michaelpq

src/bin/pg_upgrade/pg_upgrade.c

@@ -437,6 +437,7 @@ set_locale_and_encoding(void)
 	char	   *datcollate_literal;
 	char	   *datctype_literal;
 	char	   *datlocale_literal = NULL;
+	char	   *datlocale_src;
 	DbLocaleInfo *locale = old_cluster.template0;
 
 	prep_status("Setting locale and encoding for new cluster");
@@ -450,12 +451,10 @@ set_locale_and_encoding(void)
 	datctype_literal = PQescapeLiteral(conn_new_template1,
 									   locale->db_ctype,
 									   strlen(locale->db_ctype));
-	if (locale->db_locale)
-		datlocale_literal = PQescapeLiteral(conn_new_template1,
-											locale->db_locale,
-											strlen(locale->db_locale));
-	else
-		datlocale_literal = pg_strdup("NULL");
+	datlocale_src = locale->db_locale ? locale->db_locale : "NULL";
+	datlocale_literal = PQescapeLiteral(conn_new_template1,
+										datlocale_src,
+										strlen(datlocale_src));
 
 	/* update template0 in new cluster */
 	if (GET_MAJOR_VERSION(new_cluster.major_version) >= 1700)