Fix use of uninitialized memory in test_rsa_oaep

Fix use of uninitialized memory in test_rsa_oaep

48f1739600f33c92387debce2002acec6e365f1d did not convert the RSA OAEP
tests correctly. The corrupted ciphertext and truncation tests were
really decrypting uninitialized memory, rather than the sample
ciphertext. This results in an error in tools like MSan.

The test is somewhat roundabout. In the original version, before the
conversion, ctext_ex was an OAEP test vector from key1(), etc.,
functions. The test would:

1. Encrypt ptext_ex as ctext.
2. Decrypt ctext and check it gives ptext_ex.
3. Decrypt ctext_ex and check it gives ptext_ex.
4. Try corrupted and truncated versions of ctext.

48f1739600f33c92387debce2002acec6e365f1d then moved steps 1 and 2 into
test_rsa_simple, which meant ctext is no longer available for step 4. It
then mistakenly left the variable around, but uninitialized, so the test
wasn't testing anything. (Confusingly, test_rsa_simple outputs ctext_ex
to the caller, but doesn't do anything with it. The ctext_ex output is
also only usable for OAEP, not PKCS#1 v1.5.)

It doesn't really matter whether we use ctext or ctext_ex for step 4, so
this PR fixes it by using ctext_ex instead.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15950)

(cherry picked from commit 36a4637e158508f5d2fb7750e4870888072a56f9)