Add missing return value checks

The function DH_check_pub_key() was missing some return value checks in
some calls to BN functions.

RT#4278
Reviewed-by: NAndy Polyakov <appro@openssl.org>

crypto/dh/dh_check.c

@@ -151,13 +151,12 @@ int DH_check_pub_key(const DH *dh, const BIGNUM *pub_key, int *ret)
         goto err;
     BN_CTX_start(ctx);
     tmp = BN_CTX_get(ctx);
-    if (tmp == NULL)
+    if (tmp == NULL || !BN_set_word(tmp, 1))
         goto err;
-    BN_set_word(tmp, 1);
     if (BN_cmp(pub_key, tmp) <= 0)
         *ret |= DH_CHECK_PUBKEY_TOO_SMALL;
-    BN_copy(tmp, dh->p);
-    BN_sub_word(tmp, 1);
+    if (BN_copy(tmp, dh->p) == NULL || !BN_sub_word(tmp, 1))
+        goto err;
     if (BN_cmp(pub_key, tmp) >= 0)
         *ret |= DH_CHECK_PUBKEY_TOO_LARGE;