add missing check for BN_mod_inverse

Fixes #7650
Reviewed-by: NKurt Roeckx <kurt@roeckx.be>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7876)

(cherry picked from commit 55833a8de70589a5000044b6291e190f5a3826ae)

crypto/rsa/rsa_x931g.c

@@ -128,6 +128,8 @@ int RSA_X931_derive_ex(RSA *rsa, BIGNUM *p1, BIGNUM *p2, BIGNUM *q1,
 
     /* calculate inverse of q mod p */
     rsa->iqmp = BN_mod_inverse(NULL, rsa->q, rsa->p, ctx2);
+    if (rsa->iqmp == NULL)
+        goto err;
 
     ret = 1;
  err: