Check a return value in the SRP code

Spotted by OSTIF audit

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8019)

(cherry picked from commit 0a5bda639f)

crypto/srp/srp_vfy.c

@@ -691,6 +691,8 @@ int SRP_create_verifier_BN(const char *user, const char *pass, BIGNUM **salt,
     }
 
     x = SRP_Calc_x(salttmp, user, pass);
+    if (x == NULL)
+        goto err;
 
     *verifier = BN_new();
     if (*verifier == NULL)