OPENSSL_cleanse some buffers.

See upstream's 5292833132cc863b66574fe2bbf55e4b2eff7949. Syncing just to
reduce the diff for the time being.

Change-Id: I0992d538b283d7348ef1d993973291f5416edce6
Reviewed-on: https://boringssl-review.googlesource.com/18804
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/pem/pem_pk8.c

@@ -176,6 +176,7 @@ EVP_PKEY *d2i_PKCS8PrivateKey_bio(BIO *bp, EVP_PKEY **x, pem_password_cb *cb,
     }
     p8inf = PKCS8_decrypt(p8, psbuf, klen);
     X509_SIG_free(p8);
+    OPENSSL_cleanse(psbuf, klen);
     if (!p8inf)
         return NULL;
     ret = EVP_PKCS82PKEY(p8inf);

crypto/pem/pem_pkey.c

@@ -114,6 +114,7 @@ EVP_PKEY *PEM_read_bio_PrivateKey(BIO *bp, EVP_PKEY **x, pem_password_cb *cb,
         }
         p8inf = PKCS8_decrypt(p8, psbuf, klen);
         X509_SIG_free(p8);
+        OPENSSL_cleanse(psbuf, klen);
         if (!p8inf)
             goto p8err;
         ret = EVP_PKCS82PKEY(p8inf);