tests: Fix two memory leaks

Although both leaks are not really important, fix them
to avoid cppcheck warnings:

tests/linux-test.c:433: error: Memory leak: stack1
tests/linux-test.c:433: error: Memory leak: stack2

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
