"EC_POINT_invert" was checking "dbl" function pointer instead of "invert".

PR#2569
Reviewed-by: NRich Salz <rsalz@openssl.org>

crypto/ec/ec_lib.c

@@ -945,7 +945,7 @@ int EC_POINT_dbl(const EC_GROUP *group, EC_POINT *r, const EC_POINT *a, BN_CTX *
 
 int EC_POINT_invert(const EC_GROUP *group, EC_POINT *a, BN_CTX *ctx)
 	{
-	if (group->meth->dbl == 0)
+	if (group->meth->invert == 0)
 		{
 		ECerr(EC_F_EC_POINT_INVERT, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
 		return 0;