Avoid potential overflow to the sign bit when shifting left 24 places

Although there are platforms where int is 64 bit, 2GiB large BIGNUMs
instead of 4GiB should be "big enough for everybody".

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from #11857)

crypto/bn/bn_mpi.c

@@ -45,7 +45,7 @@ BIGNUM *BN_mpi2bn(const unsigned char *d, int n, BIGNUM *ain)
     int neg = 0;
     BIGNUM *a = NULL;
 
-    if (n < 4) {
+    if (n < 4 || (d[0] & 0x80) != 0) {
         BNerr(BN_F_BN_MPI2BN, BN_R_INVALID_LENGTH);
         return NULL;
     }