Fix another possible crash in rsa_ossl_mod_exp.

Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3675)

crypto/rsa/rsa_ossl.c

@@ -608,6 +608,8 @@ static int rsa_ossl_mod_exp(BIGNUM *r0, const BIGNUM *I, RSA *rsa, BN_CTX *ctx)
     r1 = BN_CTX_get(ctx);
     m1 = BN_CTX_get(ctx);
     vrfy = BN_CTX_get(ctx);
+    if (vrfy == NULL)
+        goto err;
 
     {
         BIGNUM *p = BN_new(), *q = BN_new();