cg: fix calls to rand

For many types of identifier, a di_id of 0 means "undefined".  But for
subrs, di_id 0 is DIF_SUBR_RAND, a valid (albeit currently
unimplemented) subr.

Fixes several assertion failures running the testsute.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

libdtrace/dt_cg.c

@@ -3325,7 +3325,7 @@ dt_cg_call_subr(dt_node_t *dnp, dt_irlist_t *dlp, dt_regset_t *drp)
 					"required)\n",
 			dt_idkind_name(idp->di_kind), idp->di_name);
 
-	assert(idp->di_id > 0 && idp->di_id <= DIF_SUBR_MAX);
+	assert(idp->di_id <= DIF_SUBR_MAX);
 
 	fun = _dt_cg_subr[idp->di_id];
 	if (fun == NULL)