Fix cyclic import/export segfault

Before this commit it segfaulted, now it throws a SyntaxError.
That's still not correct behavior but better than segfaulting.
To be continued.

Refs: quickjs-ng#567

quickjs.c

@@ -26774,6 +26774,11 @@ static JSValue js_build_module_ns(JSContext *ctx, JSModuleDef *m)
         case EXPORTED_NAME_NORMAL:
             {
                 JSVarRef *var_ref = en->u.var_ref;
+                if (!var_ref) {
+                    js_resolve_export_throw_error(ctx, JS_RESOLVE_RES_CIRCULAR,
+                                                  m, en->export_name);
+                    goto fail;
+                }
                 pr = add_property(ctx, p, en->export_name,
                                   JS_PROP_ENUMERABLE | JS_PROP_WRITABLE |
                                   JS_PROP_VARREF);

tests.conf

@@ -4,5 +4,6 @@ verbose=yes
 testdir=tests
 
 [exclude]
+tests/fixture_cyclic_import.js
 tests/microbench.js
 tests/test_worker_module.js

tests/fixture_cyclic_import.js

@@ -0,0 +1,2 @@
+import * as a from "./test_cyclic_import.js"
+export function f(x) { return 2 * a.g(x) }

tests/test_cyclic_import.js

@@ -0,0 +1,12 @@
+/*---
+negative:
+  phase: resolution
+  type: SyntaxError
+---*/
+// FIXME(bnoordhuis) shouldn't throw SyntaxError but that's still better
+// than segfaulting, see https://github.com/quickjs-ng/quickjs/issues/567
+import {assert} from "./assert.js"
+import {f} from "./fixture_cyclic_import.js"
+export {f}
+export function g(x) { return x + 1 }
+assert(f(1), 4)