ww: preserve fallible graph allocation in self-host

selfhost/cmd/ww/main.ww

@@ -993,6 +993,18 @@ fn sepallocptrs(cap: i32) ([]*u8 | nomem) = {
 	return value;
 };
 
+fn sepallocgraph(pkg: []seppkg, context: []sepcontext) (*sepgraph | nomem) = {
+	let value: *sepgraph = alloc(sepgraph{
+		pkg = pkg,
+		n = 0,
+		context = context,
+		ncontext = 0,
+		supportcontext = -1,
+		identityfailed = false,
+	})?;
+	return value;
+};
+
 fn sepmakeints(count: i32, out: *[]i32) bool = {
 	if (count < 0) {
 		sepfailsize();
@@ -3613,14 +3625,8 @@ fn buildonesepimpl(selfdir: *u8, src: *u8, entryisdir: i32,
 
 	let pkgslot: []seppkg;
 	let contextslot: []sepcontext;
-	let graphallocation: (*sepgraph | nomem) = alloc(sepgraph{
-		pkg = pkgslot,
-		n = 0,
-		context = contextslot,
-		ncontext = 0,
-		supportcontext = -1,
-		identityfailed = false,
-	});
+	let graphallocation: (*sepgraph | nomem) = sepallocgraph(pkgslot,
+		contextslot);
 	let g: *sepgraph;
 	match (graphallocation) {
 	case let value: *sepgraph => g = value;