Fix use of uninitialized memory in test_rsa_oaep

48f1739 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.

48f1739 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 openssl#15950)

test/rsa_test.c

@@ -283,7 +283,6 @@ static int test_rsa_oaep(int idx)
     int ret = 0;
     RSA *key = NULL;
     unsigned char ptext[256];
-    unsigned char ctext[256];
     static unsigned char ptext_ex[] = "\x54\x85\x9b\x34\x2c\x49\xea\x2a";
     unsigned char ctext_ex[256];
     int plen;
@@ -305,17 +304,17 @@ static int test_rsa_oaep(int idx)
 
     /* Try decrypting corrupted ciphertexts. */
     for (n = 0; n < clen; ++n) {
-        ctext[n] ^= 1;
-        num = RSA_private_decrypt(clen, ctext, ptext, key,
+        ctext_ex[n] ^= 1;
+        num = RSA_private_decrypt(clen, ctext_ex, ptext, key,
                                       RSA_PKCS1_OAEP_PADDING);
         if (!TEST_int_le(num, 0))
             goto err;
-        ctext[n] ^= 1;
+        ctext_ex[n] ^= 1;
     }
 
     /* Test truncated ciphertexts, as well as negative length. */
     for (n = -1; n < clen; ++n) {
-        num = RSA_private_decrypt(n, ctext, ptext, key,
+        num = RSA_private_decrypt(n, ctext_ex, ptext, key,
                                   RSA_PKCS1_OAEP_PADDING);
         if (!TEST_int_le(num, 0))
             goto err;