Fix SRP buffer overrun vulnerability.

Invalid parameters passed to the SRP code can be overrun an internal
buffer. Add sanity check that g, A, B < N to SRP code.

Thanks to Sean Devlin and Watson Ladd of Cryptography Services, NCC
Group for reporting this issue.

crypto/srp/srp_lib.c

@@ -89,6 +89,9 @@ static BIGNUM *srp_Calc_k(BIGNUM *N, BIGNUM *g)
 	int longg ;
 	int longN = BN_num_bytes(N);
 
+	if (BN_ucmp(g, N) >= 0)
+		return NULL;
+
 	if ((tmp = OPENSSL_malloc(longN)) == NULL)
 		return NULL;
 	BN_bn2bin(N,tmp) ;
@@ -121,6 +124,9 @@ BIGNUM *SRP_Calc_u(BIGNUM *A, BIGNUM *B, BIGNUM *N)
 	if ((A == NULL) ||(B == NULL) || (N == NULL))
 		return NULL;
 
+	if (BN_ucmp(A, N) >= 0 || BN_ucmp(B, N) >= 0)
+		return NULL;
+
 	longN= BN_num_bytes(N);
 
 	if ((cAB = OPENSSL_malloc(2*longN)) == NULL)