test: aes: fix memleak

In the first version, the result of malloc is checked
with ut_assertnonnull. But on a fail, this macro exit
the function, so previously malloc are not freed.

So to avoid a memleak, we don't use ut_assertnonnull,
but simply check the return of malloc. If one has failed,
we freed all the allocated memory and quit the function.

Reported-by: Coverity (CID: 284403)
Reported-by: Coverity (CID: 284404)
Reported-by: Coverity (CID: 284405)
Reported-by: Coverity (CID: 284406)
Reported-by: Coverity (CID: 284407)
Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>