auth-cfg: Fix crash after several reauthentications with multiple authentication rounds

Due to the issue described in c641974, purge() inadvertently destroyed
CA certificates that should have been kept (while the pointer to these
objects remained in the array).  This lead to incorrect reference counts
and after a few reauthentications with multiple authentication rounds,
which cause calls to purge(TRUE), to crashes.

src/libstrongswan/credentials/auth_cfg.c

@@ -998,8 +998,8 @@ METHOD(auth_cfg_t, purge, void,
 	{
 		if (!keep_ca || entry->type != AUTH_RULE_CA_CERT)
 		{
-			array_remove_at(this->entries, enumerator);
 			destroy_entry_value(entry);
+			array_remove_at(this->entries, enumerator);
 		}
 	}
 	enumerator->destroy(enumerator);