Check return from BN_sub

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6823)

crypto/rsa/rsa_ossl.c

diff --git a/crypto/rsa/rsa_ossl.c b/crypto/rsa/rsa_ossl.c
index feb1ab4..c441905 100644
--- a/crypto/rsa/rsa_ossl.c
+++ b/crypto/rsa/rsa_ossl.c
@@ -339,7 +339,8 @@
             goto err;
 
     if (padding == RSA_X931_PADDING) {
-        BN_sub(f, rsa->n, ret);
+        if (!BN_sub(f, rsa->n, ret))
+            goto err;
         if (BN_cmp(ret, f) > 0)
             res = f;
         else