Fix error handling with uk_posix_memalign()

Fix error handling with uk_posix_memalign()

In the cases of failed allocation, `uk_posix_memalign()` is returning
an errno number but does not change `memptr`. Instead of checking
`memptr` being NULL, we test failures with `uk_posix_memalign()`'s
return codes.

Signed-off-by: Simon Kuenzer <simon.kuenzer@neclab.eu>
Reviewed-by: Felipe Huici <felipe.huici@neclab.eu>