Add missing range checks on number of multi primes in rsa_ossl_mod_exp

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4862)

crypto/rsa/rsa_ossl.c

diff --git a/crypto/rsa/rsa_ossl.c b/crypto/rsa/rsa_ossl.c
index ced11ad..f7b5cdc 100644
--- a/crypto/rsa/rsa_ossl.c
+++ b/crypto/rsa/rsa_ossl.c
@@ -604,7 +604,7 @@
 
 static int rsa_ossl_mod_exp(BIGNUM *r0, const BIGNUM *I, RSA *rsa, BN_CTX *ctx)
 {
-    BIGNUM *r1, *m1, *vrfy, *r2, *m[RSA_MAX_PRIME_NUM];
+    BIGNUM *r1, *m1, *vrfy, *r2, *m[RSA_MAX_PRIME_NUM - 2];
     int ret = 0, i, ex_primes = 0;
     RSA_PRIME_INFO *pinfo;
 
@@ -618,7 +618,8 @@
         goto err;
 
     if (rsa->version == RSA_ASN1_VERSION_MULTI
-        && (ex_primes = sk_RSA_PRIME_INFO_num(rsa->prime_infos)) <= 0)
+        && ((ex_primes = sk_RSA_PRIME_INFO_num(rsa->prime_infos)) <= 0
+             || ex_primes > RSA_MAX_PRIME_NUM - 2))
         goto err;
 
     {