Fix EC_GROUP_new_from_ecparameters to check the base length

Check that there's at least one byte in params->base before trying to
read it.

CVE-2021-3712

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/ec/ec_asn1.c

@@ -699,7 +699,8 @@ EC_GROUP *EC_GROUP_new_from_ecparameters(const ECPARAMETERS *params)
 
     if (params->order == NULL
             || params->base == NULL
-            || params->base->data == NULL) {
+            || params->base->data == NULL
+            || params->base->length == 0) {
         ERR_raise(ERR_LIB_EC, EC_R_ASN1_ERROR);
         goto err;
     }