Fix small memory leaks in GUC checks

Follow-up commit to a9d58bfe8a.  Backpatch down to v16 where
this was added in order to keep the code consistent for future
backpatches.

Author: Tofig Aliev <t.aliev@postgrespro.ru>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/bba4313fdde9db46db279f96f3b748b1@postgrespro.ru
Backpatch-through: 16

src/backend/commands/variable.c

@@ -1082,6 +1082,8 @@ check_application_name(char **newval, void **extra, GucSource source)
 		return false;
 	}
 
+	guc_free(*newval);
+
 	pfree(clean);
 	*newval = ret;
 	return true;
@@ -1118,6 +1120,8 @@ check_cluster_name(char **newval, void **extra, GucSource source)
 		return false;
 	}
 
+	guc_free(*newval);
+
 	pfree(clean);
 	*newval = ret;
 	return true;