Fix unintentional 'NULL' string literal in pg_upgrade.

Introduced in 2a083ab807.

Note: backport of commit 945126234b, which was missed at the time.

Discussion: https://postgr.es/m/e852442da35b4f31acc600ed98bbee0f12e65e0c.camel@j-davis.com
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Backpatch-through: 16

src/bin/pg_upgrade/pg_upgrade.c

@@ -408,7 +408,6 @@ 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");
@@ -422,10 +421,13 @@ set_locale_and_encoding(void)
 	datctype_literal = PQescapeLiteral(conn_new_template1,
 									   locale->db_ctype,
 									   strlen(locale->db_ctype));
-	datlocale_src = locale->db_locale ? locale->db_locale : "NULL";
-	datlocale_literal = PQescapeLiteral(conn_new_template1,
-										datlocale_src,
-										strlen(datlocale_src));
+
+	if (locale->db_locale)
+		datlocale_literal = PQescapeLiteral(conn_new_template1,
+											locale->db_locale,
+											strlen(locale->db_locale));
+	else
+		datlocale_literal = "NULL";
 
 	/* update template0 in new cluster */
 	if (GET_MAJOR_VERSION(new_cluster.major_version) >= 1700)
@@ -469,7 +471,8 @@ set_locale_and_encoding(void)
 
 	PQfreemem(datcollate_literal);
 	PQfreemem(datctype_literal);
-	PQfreemem(datlocale_literal);
+	if (locale->db_locale)
+		PQfreemem(datlocale_literal);
 
 	PQfinish(conn_new_template1);