pg_upgrade: Fix memory leak in check_for_unicode_update().

This function was initializing the "task" variable before a couple
of early returns.  To fix, postpone the initialization until just
before it's needed.

Per Coverity.

Discussion: https://postgr.es/m/Z_KMsUH2-FEbiNjC%40nathan

src/bin/pg_upgrade/check.c

@@ -1825,7 +1825,7 @@ static void
 check_for_unicode_update(ClusterInfo *cluster)
 {
 	UpgradeTaskReport report;
-	UpgradeTask *task = upgrade_task_create();
+	UpgradeTask *task;
 	const char *query;
 
 	/*
@@ -1920,6 +1920,7 @@ check_for_unicode_update(ClusterInfo *cluster)
 		"        d.datname = current_database() AND "
 		"        d.encoding = pg_char_to_encoding('UTF8');";
 
+	task = upgrade_task_create();
 	upgrade_task_add_step(task, query,
 						  process_unicode_update,
 						  true, &report);