config: free the list of included files properly

refs #2381

src/knot/conf/cf-lex.l

@@ -344,8 +344,10 @@ hmac-sha512     { lval.alg = KNOT_TSIG_ALG_HMAC_SHA512; return TSIG_ALGO_NAME; }
 	char *name = names_stack_pop(yyextra);
 	free(name);
 	yypop_buffer_state(yyscanner);
-	if (!YY_CURRENT_BUFFER)
+	if (!YY_CURRENT_BUFFER) {
+		free(yyextra);
 		return END;
+	}
 }
 
 <include>{BLANK}+