Clear buffer in PEM_write_bio

Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5814)

crypto/pem/pem_lib.c

@@ -661,7 +661,7 @@ int PEM_write_bio(BIO *bp, const char *name, const char *header,
     if (retval == 0)
         PEMerr(PEM_F_PEM_WRITE_BIO, reason);
     EVP_ENCODE_CTX_free(ctx);
-    OPENSSL_free(buf);
+    OPENSSL_clear_free(buf, PEM_BUFSIZE * 8);
     return retval;
 }