Fix assertion failure in CREATE DATABASE

An assertion would fail when creating a database with libc locale
provider from a template database with icu locale provider.

Reported-by: Marina Polyakova <m.polyakova@postgrespro.ru>
Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/f385ba25e7f8be427b8c582e5cca7d79%40postgrespro.ru#515a31c5429d6d37ad1d5c9d66962a1e

src/backend/commands/dbcommands.c

@@ -1012,10 +1012,10 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 		dbcollate = src_collate;
 	if (dbctype == NULL)
 		dbctype = src_ctype;
-	if (dbiculocale == NULL)
-		dbiculocale = src_iculocale;
 	if (dblocprovider == '\0')
 		dblocprovider = src_locprovider;
+	if (dbiculocale == NULL && dblocprovider == COLLPROVIDER_ICU)
+		dbiculocale = src_iculocale;
 
 	/* Some encodings are client only */
 	if (!PG_VALID_BE_ENCODING(encoding))

src/bin/scripts/t/020_createdb.pl

@@ -49,6 +49,15 @@ if ($ENV{with_icu} eq 'yes')
 			'--icu-locale=@colNumeric=lower', 'foobarX'
 		],
 		'fails for invalid ICU locale');
+
+	# additional node, which uses the icu provider
+	my $node2 = PostgreSQL::Test::Cluster->new('icu');
+	$node2->init(extra => ['--locale-provider=icu', '--icu-locale=en']);
+	$node2->start;
+
+	$node2->command_ok(
+		[ 'createdb', '-T', 'template0', '--locale-provider=libc', 'foobar55' ],
+		'create database with libc provider from template database with icu provider');
 }
 else
 {