fix: js atom null

[stefan-gorules]

Hi, I've ran into an issue on Linux where calling Module::import resulted in an error with UTF8. After closer inspection I've noticed that Ctx::script_or_module_name generated what appeared to be pseudo-random values. I've dug into JS_GetScriptOrModuleName in c:

/* return JS_ATOM_NULL if the name cannot be found. Only works with
   not striped bytecode functions. */
JSAtom JS_GetScriptOrModuleName(JSContext *ctx, int n_stack_levels)
{
    JSStackFrame *sf;
    JSFunctionBytecode *b;
    JSObject *p;
    /* XXX: currently we just use the filename of the englobing
       function from the debug info. May need to add a ScriptOrModule
       info in JSFunctionBytecode. */
    sf = ctx->rt->current_stack_frame;
    if (!sf)
        return JS_ATOM_NULL;
    while (n_stack_levels-- > 0) {
        sf = sf->prev_frame;
        if (!sf)
            return JS_ATOM_NULL;
    }
    for(;;) {
        if (JS_VALUE_GET_TAG(sf->cur_func) != JS_TAG_OBJECT)
            return JS_ATOM_NULL;
        p = JS_VALUE_GET_OBJ(sf->cur_func);
        if (!js_class_has_bytecode(p->class_id))
            return JS_ATOM_NULL;
        b = p->u.func.function_bytecode;
        if (!b->is_direct_or_indirect_eval) {
            if (!b->has_debug)
                return JS_ATOM_NULL;
            return JS_DupAtom(ctx, b->debug.filename);
        } else {
            sf = sf->prev_frame;
            if (!sf)
                return JS_ATOM_NULL;
        }
    }
}

We can see that JS_ATOM_NULL is returned in case script name cannot be resolved. This is not the same value as the one currently being checked against (JS_ATOM_null = 1, __JS_ATOM_NULL = 0). As is defined in .h.

#define JS_ATOM_NULL 0

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@@             Coverage Diff             @@
##           master     #333       +/-   ##
===========================================
- Coverage   68.31%   55.83%   -12.48%     
===========================================
  Files          83       83               
  Lines       12237    12237               
===========================================
- Hits         8360     6833     -1527     
- Misses       3877     5404     +1527     

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[DelSkayn]

Good catch! Thanks for the PR!